Yahoo CEO Carol Bartz is making the most substantive changes in her exec ranks since she did a massive restructuring of its staff in late February, according to sources close to the situation.

“She is continuing to clean the place up,” said one top exec about the moves, which are likely to be announced internally tomorrow.

Among the shifts in management will be filling the slot left by the departure of North American Audience head Jeff Dossett in May.

UPDATE: Sources say Yahoo’s head of mobile, David Ko, will get the job of top Audience exec, although it is not clear if he will have the same portfolio has former media heads at Yahoo.

Since Dossett left, his job has been split between Jimmy Pitaro, who runs Vertical Audience Experiences, and Tim Mayer, who is in charge of Search & Social Applications. They both currently report to U.S. EVP Hilary Schneider.

The job of Audience head is a key role, given that Yahoo’s powerful media properties are among its most valuable assets. In recent months, Yahoo has made some major changes in the way it creates its juggernaut News property.

Also to be filled is the job being done by Corporate Partnership SVP Jim Schinella, who, as BoomTown previously reported, is set to leave at the end of the year.

I could not determine who will take Schinella’s job, inside or out.

Interestingly, Yahoo has yet to name an international head.

Sources said the company had filled the position, using a headhunter, but the London-based media exec candidate backed out at the last minute. That meant Yahoo had to restart its search.

There might also be other top exec changes, all part of Bartz’s consolidation of power at Yahoo. She has named a spate of new top execs from outside, but has also kept some from the regime of former CEO and co-founder Jerry Yang.

These staffing moves have come even as a stream of execs continued to depart the Silicon Valley Internet giant, including, most recently, Mike Walrath, who was SVP of advertising strategy. Walrath had led Right Media, the online ad exchange Yahoo bought for $680 million in 2007.

Walrath was widely expected to leave Yahoo in July, at the completion of his earnout from the acquisition, sources said, so the move was more sudden than expected internally.

Sources noted that Bartz moved Walrath’s departure forward in order to announce a new strategy for Right Media focused on premium publishers and to dump those ad networks and publishers of lesser ilk.

Whether this will stop the competitive onslaught in the ad exchange space is an open question given that Google has entered the fray significantly and that Facebook is widely expected to bolster its efforts.

Lastly, several sources said that there are also likely to be more changes on Yahoo’s board, which has seen the departure of two members recently.

In September, Maggie Wilderotter said she would leave the board by year’s end. And former Yahoo nemesis and investor Carl Icahn left the board in late October.

Whether Yahoo will replace them or keep its current size of 10 directors is not clear.

Also possible, several sources said, would be Bartz taking the chairman title, which is currently held by Roy Bostock. Bostock, along with Yang, played a key role in its botched takeover battle with Microsoft (MSFT).

Bartz finally successfully struck a sweeping search and advertising partnership with the software giant this summer, which is moving closer to being launched.

The departure of Dave Karnstedt, who took over last year when longtime Yahoo ad sales exec Wenda Millard left Yahoo in the first of many controversial partings, has been long rumored internally.

(In fact, I have driven one of Yahoo’s PR people crazy in recent months trying to verify a persistent tip I had been getting that he was headed out the door.)

Karnstedt will be joining Redpoint Ventures, a Silicon Valley venture firm, as an executive-in-residence.

And, in a rejiggering and addition of duties at Yahoo (YHOO), Karnstedt’s job and more is going to Joanne Bradford (pictured here), a longtime and well-known Microsoft (MSFT) exec who decamped from the software giant to helm national ad sales at trendy ad services start-up Spot Runner just six months ago.

There have been rumors swirling that Bradford was unhappy at the smaller company after working at the giant Microsoft.

She was EVP of National Marketing Services, focused on national advertisers, for Spot Runner, joining in a high-profile move in March. Previous to Spot Runner, Bradford was a VP and chief media officer of MSN Media Network, and had worked at BusinessWeek before that.

In any case, the move will be seen as a blow to Spot Runner, which recently did some unusual layoffs, despite receiving a large slug of cash from investors.

(Here is a post and video I did on a recent trip to Spot Runner, including an interview with its CEO Nick Grouf.)

“I am going back to my entrepreneurial, build-something roots,” Bradford told me at the time she joined Spot Runner. “There is such inefficiency in buying and selling of advertising and someone has to solve that, both for big companies and small ones.”

Well, welcome to Yahoo, Joanne, which could use a little efficiency in its buying and selling of ad sales!

Seriously, Bradford will now will take over as SVP of U.S. revenue and market development at Yahoo at a very dicey time.

Besides facing a withering U.S. economy, a weakened stock price after the takeover attempt by Microsoft and ensuing mess related to it, it was revealed that the Justice Department might block the deal Yahoo recently struck to outsource some of its ad sales to Google (GOOG).

Yahoo said that in this newly created role Bradford will oversee sales, market development for advertisers, small business and HotJobs. She will report to Hilary Schneider, EVP of Yahoo’s U.S unit.

Karnstedt, whom I interviewed when he first took over ad sales a little more than a year ago, is leaving to pursue other opportunities.

In Silicon Valley, that means the inevitable stop at a VC firm. Hence, Redpoint!

Interestingly, he joins former Ask.com head Jim Lanzone at Redpoint, while former Yahoo execs Jeff Weiner (Accel Partners and Benchmark Capital) and David Goldberg (Benchmark) also landed cushy EIR gigs after leaving Yahoo.

Karnstedt had been SVP of U.S. sales at Yahoo and had apparently resigned from the company earlier this summer (thanks for not confirming that when I asked so many times, Yahoo!)

With Yahoo seven years, he was charged with the difficult task of integrating Yahoo’s search, display, Blue Lithium and Right Media sales teams.

And while Karnstedt was well liked, many complained that the longtime online ad techie was not enough of a gregarious and schmoozy ad sales exec, with deep relationships on Madison Avenue, as Millard–and Bradford–surely are.

As I wrote in Aug. 2007, after an interview with him at Yahoo’s New York offices:

I made the point to Dave (he is the kind of guy you can call Dave, as you can see pictured here) that an ad guy needs to sell himself, but to no avail, so we press on in text. Nonetheless, let me set the visual scene:

Nicest guy you ever want to meet walks into nondescript room, wearing khaki-oxford-jacket Internet uniform 101. Declares Yahoo is going to kick some advertising butt in the nicest possible way. It is revealed this nice guy has been around the Web block for quite a while. Much chitter-chatter ensues. Cut to my clear-as-Fiji-water observation that nice guy, as nice as he is, has his work cut out for him.”

And now, more than ever in Yahoo’s key ad market, so does Bradford.

But I seem to be in violent agreement with Sanford Bernstein analyst Jeff Lindsay of late–at least with a recent report he just did calling for Yahoo to abandon its slower-moving strategies and get much, much bolder much, much quicker.

Suggestions by Lindsay included outsourcing its search business, making deep cuts in staff and also doubling down on its bets in its ad network businesses like Right Media.

(Frankly, I’d just like to see the company immediately-if-not-sooner roll out innovative email features like CEO and Co-Founder Jerry Yang showed at the Consumer Electronics Show last week.)

If you recall, Lindsay penned a previous report last October on the worth of Yahoo by parsing out its various assets. It was instructive in its focus on the value of Yahoo’s somewhat liquid holdings like investments compared to its core business.

The message at the time: Yahoo had some valuable assets–such as its stake in China’s Alibaba.com–and its stock did not reflect these gems. It even suggested the company be split into parts to unlock value.

But his most recent piece is less sanguine–a kind of flip side to the first, noting that the operations side of the business was not up to snuff, causing the valuation of Yahoo to fall. Bernstein blames Yahoo’s too-careful management, as well as its declining share of the search market.

Whatever you think about Yahoo, its still lackluster stock price–it hovers in the low $20-range–make reports like Lindsay’s interesting reading. See also this Motley Fool report yesterday, naming Yahoo the “Worst Stock for 2008.”

Nonetheless, all this bearishness could foretell some bullishness on Yahoo, which appears to simply refuse to move faster than it wants to.

Consider a largely positive piece on Yahoo’s fine-tuning of its business in the New York Times yesterday, which chronicled Yang’s turtle-versus-hare approach to the company’s future.

The article quoted Yang’s I’m-still-here intro to the CES speech (pictured here in this AP shot by Paul Sakuma), which kind of says all you need to know: “I’m guessing that a lot of you are here today to see what the new look and new face of Yahoo is all about…well, I’m sorry to disappoint you. It’s still the same old face. I’ve been around since the beginning.”

And, I have no doubt, until the bitter, sweet or even bittersweet end.

That’s more than 10 times its revenue last year, and, yipes, close to 50 times its cash flow. And that is double what the Seattle-based parent company to Avenue A | Razorfish was worth on the public market just before the acquisition, a figure that has already been bid up by all the recent activity in the market.

That includes Google’s $3.1 billion bid for DoubleClick, Yahoo’s $680 million to buy the rest of Right Media and WPP Group’s acquisition this week of 24/7 Real Media for $649 million. And, by the way, AOL bought a German-based online ad company called Adtech this week, too.

If you don’t know what to make of all this, consider yourself in the majority, as these prices seem–let’s just come out and say it, as we are not investment bankers–insane. In fact, the general Internet acquisition market feels to me a lot like the wacky IPO market back in the height of the bubble, where you were often slack-jawed by the rising stock prices for companies with no visible means of comparable growth.

But before I get going on that rant, at least the big players are overpaying in a market that I think we can all agree is one that is just at its most early stages. Here’s why:

1. Spending by big advertisers online lags well behind what many call “audience engagement.” In other words, time spent on the Web has obviously been growing and taking share away from traditional media. But spending online, though fast growing at about $20 billion this year, has not kept the same pace.

2. The time to act, then, is now, to lock up any and all available assets in this space, especially ones that give the buyer a big market share and critical mass. The three biggest online ad players, Google, Microsoft and Yahoo, have snapped up the three biggest independent online ad agencies.

3. As more ad spending shifts online, the ability to have expertise and to innovate quickly will become critical. What all these companies are buying–besides stronger relationships with advertising clients–are people and experience.

4. Most of all, there was no way Microsoft was not going to answer Google after it bought DoubleClick, especially if it wants (and it does) to stay competitive with the search giant in the online ad market. Given that its talks with Yahoo about some sort of partnership (as I have said before–please don’t) have not borne fruit (as eager as, I am sure, Microsoft ’s Steve Ballmer would like to make that announcement), such a move by the company seemed inevitable.

Could they resist? I think not.

Please see this disclosure related to me and Google.