The yearly meeting of the media and technology elite in Sun Valley, Idaho, is known as a venue where moguls wear blue jeans and big deals are hatched over scenic bicycle rides, leisurely strolls and chats down by the pond.

At this year's event, which Allen & Co, the investment bank, hosted this week, the biggest deal brewing - Microsoft or one of the other attendees buying Yahoo! - may actually be one forced by a feisty billionaire who is definitely not on the invite list: Carl Icahn.

In the context of Silicon Valley, Web 2.0, and the latest “new thing”, Icahn can seem ludicrous: aged 72, a reformed 1980s raider, working from dark, wood-panelled offices in midtown Manhattan like a set from Wall Street. He does not use e-mail. His energy drink is a Martini. But he was the talk of Idaho.

After Microsoft dropped its $42 billion takeover offer in May, Icahn swooped and bought 5 per cent of Yahoo!'s stock. Now he is mounting a proxy fight at the company's annual meeting on August 1, proposing to replace its nine-member board with his own people. He was viewed as a peripheral irritant in the Yahoo! saga with no chance of winning the board until he published an open letter on Monday that none of the moguls could miss. In it, Icahn disclosed that he had spoken several times to Steve Ballmer, the Microsoft chief executive, over the previous week: “Several of our conversations have lasted as long as an hour,” he wrote, emphasising that it was a real get-to-know-you sort of thing.

During these chats, Ballmer “made it abundantly clear” that he is still willing to reopen talks to do a deal with Yahoo! - either for the whole company or its search business - but only if Yahoo! has a new board of directors. Microsoft backed up Icahn's provocative account, saying: “While of course there can be no assurance of a future transaction, we will be prepared to enter into discussions immediately after Yahoo!'s shareholder meeting if a new board is elected.”

Icahn is, in the words of one chief executive who has tangled with him, “a savant”. That is, he has a brilliant touch for ferreting-out weakness in big corporate situations and capitalising on it to make smart, usually short-term investments through his trademark brand of trash-talking agitation and activism. (It is no fluke that Icahn is the richest man in New York City, a fortune built in part on his legendary raids on companies such as TWA, USX and Texaco.) An investor who has made money alongside Icahn described him to me in the midst of his assault on Time Warner a couple of years ago as “feral and autistic” - of course he is nothing of the sort, medically speaking, but it neatly encapsulated his great white shark-like single-mindedness.

In Yahoo!'s case, Icahn may still think widgets are made in factories, but he also knows a thing or three about finance, the markets and capitalism. And he especially knows public relations and how to kick up an almighty fuss. (He has actually been more restrained than usual in his attacks on Yahoo!, falling short of his usual description of the people in charge as “morons”.)

Yahoo! rejected Icahn and made the case for keeping the current board intact. Moreover, it has counterpunched Icahn's attacks for being “ill-defined” and based on “misrepresentations”. But in recent days there have been growing rumbles that Yahoo! has tried (unsuccessfully so far) to give Icahn a couple of seats on its board to make his proxy fight go away. His new chumminess with Ballmer has clearly exasperated Yahoo!'s board, which insists that it was Microsoft, not they, which was being unreasonable.

Even while investors appreciate Icahn's ability to crystallise their dissent, they don't necessarily want him running their company. Exhibit A in this regard has been Blockbuster Entertainment, the video rental company in which Icahn won a proxy fight in 2005, and put representatives on the board, but whose shares have traded at much less than what he paid for them.

Icahn's tactics vis-à-vis Yahoo! and Microsoft are particularly clever, because they reassure investors that if they do back his intended board, they are not voting for him, but for a sale of the company or, failing that, a better search deal than the one Yahoo! has pursued with arch-rival Google. The nagging question about Icahn is whether he is the crusader for corporate democracy that he portrays himself as, or a showman and opportunist whose main interest is short-term gain at whatever cost.

In Yahoo!'s case, he has gained the ear of several big investors who believe its board fumbled Microsoft's approach. That is tempered by the worry that Ballmer - no slouch in the tough-guy department - actually has no intention of buying Yahoo! and is just using Icahn to twist the knife. “To trust Mr Icahn and his board is really a bad choice,” Yahoo!'s Jerry Yang told the Wall Street Journal.

Whatever the outcome here, it is fascinating - and no doubt horrifying to some at Yahoo! HQ in Sunnyvale, California - to imagine Icahn making an entrance in one of the company's purple-hued meeting rooms, some of which are named after Ben & Jerry's ice-cream flavours. But his Luddite roots aside, maybe it is no coincidence that Icahn's assault on Yahoo! has coincided with the recent debut of “The Icahn Report”, a blog by the man himself. “Board meetings are often a complete travesty,” he wrote in one entry. “I know because I have sat and do sit on a number of boards where I am in the minority. Because of this, today our economy is in a major crisis. Many of our companies are incapable of competing.”

Actually, maybe Icahn is becoming more web-savvy than we've given him credit for. On his web page Icahn teases readers that he will be giving more of his tangy views about Yahoo! shortly: “If you wish to be informed I invite you to subscribe. Stay tuned.”

— Richard Siklos is Editor at Large of Fortune Magazine

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