Billionaire investor Carl Icahn is at it again. The word on the street is Mr. Icahn is considering launching a proxy fight at Yahoo (YHOO, $26.56, up $1.30) after building a significant position in the company. After buying as much as 50 million shares, the plan is to replace all or part of Yahoo’s board and get Microsoft (MSFT, $29.78, down $0.10) back to the negotiation table.

In case you haven’t heard of Carl Icahn, he was once considered an “obsessive corporate raider” after scoring big with takeovers of Texaco and USX back in the ’80’s. Now he is considered an “activist shareholder” that takes minority stakes in public companies and typically pushes for change, often threatening a proxy battle, through his hedge fund.

His hedge fund, Icahn Partners, was created in late 2004 and manages about $8 billion, of which $1.5 billion is his own money. Usually what happens is that after acquiring a small stake in a company, Mr. Icahn puts pressure on the CEO’s to do a big stock buyback or spin-off assets in order to get the stock moving. And in some cases he forces the CEO out altogether.

There’s no guarantee that Microsoft will return to the bargaining table should he be successful in replacing all or part of Yahoo’s board, but either way, he’s shaking things up. However, time is limited. The deadline for a proxy fight is this Thursday as Yahoo’s shareholders have until then to nominate a board of directors.

Rick Rouse