-By Mike Shields, Mediaweek

NEW YORK Yahoo!, Microsoft and maverick investor Carl Icahn have written each other more letters this summer than most kids do at sleep-away camp. In the latest public missive, Yahoo! CEO Jerry Yang and chairman Roy Bostock warned the company's shareholders: Icahn is not your friend, and cannot be trusted.

Yang and Bostock slammed Icahn, saying his alliance with Microsoft aimed at taking over the company will "destroy stockholder value" while focusing only on their own interests, and not the best interests of Yahoo!. According to the letter, Icahn has a "strong incentive to strike any deal with Microsoft that enables him to recover his investment and get back his money quickly, even a deal that does not provide full and fair value to you. Is that in the interests of all stockholders? Clearly, it is not."

Also in the letter, the two executives essentially called out Icahn as being a Web neophyte, and someone who is not equipped to run a giant Internet company. (One recently proposed scenario has Microsoft assuming control of Yahoo!'s search business and Icahn taking over the rest of the firm.)

"Mr. Icahn and his slate lack the working knowledge of Yahoo! and its Internet business needed to do two things that are required to successfully deliver a value-enhancing transaction for Yahoo! stockholders. First, they do not have the detailed knowledge to negotiate a complex restructuring of a large, innovative high-technology company. ... Second, they do not have the hands-on experience to manage and lead Yahoo!."

Besides questioning Icahn's ability to run a Web conglomerate, Yang and Bostock also called into question the soundness of his marriage of convenience with Microsoft, calling the pair an "odd-couple collaboration between two parties with keenly different agendas."

Even as Yahoo! maintains that it is still willing to strike the right deal with Microsoft, the executives blasted the software giant much like a politician might go after an rival opponent. "Microsoft's flip-flops and inconsistencies over the past five months are so stupefying that one can only conclude that Microsoft was never fully committed to acquiring Yahoo!," said the letter.

Regardless of any back and forth invective among the three parties, things are expected to come to a head on Aug. 1, when Yahoo! holds its much-anticipated shareholder meeting.