What a stupid purchase. Seriously, Yahoo has been on it's way out for about 10 years now. Their search business is in terminal decline, and it uses Bing for it's result anyways, which loses MS 1 billion+ dollars every quarter. I can't imagine what benefit there is to one grossly mismanaged company buying another grossly mismanaged company who is also their customer.
MS buying Yahoo will work much like Yahoo buying Geocities. In 5 years, they'll be shutting 'er down and writing off the loss on their taxes. By then, Apple will be buying MS, and Android and Ubuntu will be duking it out for being the dominant desktop OS.