Yahoo today announced that they are testing out the use of Google's ad network replacing their own ad serving system. There is no reason they would be doing this other than to throw a wrench into Microsoft's plans for a hostile takeover of Yahoo. Google is happy because they will potentially get a lot more ad inventory, and also because Microsoft is gumbling about it.
In response to this latest announcement, Microsoft released a statement of their own stating that they will assess their options -- supposedly for putting a stop to any form of partnership between the two companies.
Any definitive agreement between Yahoo! and Google would consolidate over 90% of the search advertising market in Google’s hands. This would make the market far less competitive, in sharp contrast to our own proposal to acquire Yahoo! We will assess closely all of our options. Our proposal remains the only alternative put forward that offers Yahoo! shareholders full and fair value for their shares, gives every shareholder a vote on the future of the company, and enhances choice for content creators, advertisers, and consumers.
In addition to Yahoo's proposed use of Google AdSense, they are also closing in on a deal with AOL to combine internet operations. It's clear that Yahoo doesn't want any part of a Microsoft acquisition, and will do almost anything to stop it from happening.