Investor Carl Icahn moves to oust Yahoo directors

If the two companies really abandoned hope for a deal, Aggarwal reasons they would have already announced other moves indicating they were going in a new direction.

For instance, Yahoo has been discussing a possible advertising partnership with Google for weeks without agreeing to a deal. And if Microsoft weren't still interested in Yahoo, Aggarwal believes the company would have already announced another acquisition or "radical changes" in its strategy for building a more compelling Internet search engine.

In his letter, Icahn urged Yahoo's board not to enter into any transactions, such as a Google ad partnership, that might ruin the chances of a sale to Microsoft. Ballmer cited Yahoo's flirtation with Google as one of the reasons for Microsoft's bid.

Icahn has a long history of challenging corporate boards and his efforts often result in shake-ups. Most recently, he has forced major changes at Blockbuster Inc. and Motorola Inc. He also played a pivotal role in the recent $8.5 billion sale of business software maker BEA Systems Inc. to rival Oracle Corp., which dropped an earlier bid of $6.7 billion.