MS "buying" Sony would be awful, all they would do is gut the Playstation brand/legacy and erase it from the market to allow the XBox brand to have full sway over the "hardcore" market. You're beyond naive if you think they'd have any need for the Playstation if they ever got control of it, it'd always be the red headed adopted child, the XBox would be the "real" child. They'd maybe keep the Uncharted and Gran Turismo brands until they showed declining sales, but every other Sony franchise would likely be thrown under the rug. Look what they've done to Rare.
They'd also likely just bail on the Japanese industry period.
The Microsoft-Nokia "partnership" is going horribly by the way. Nokia is in big trouble.
Sony + Nintendo (with perhaps a little bit of Google in there) would make a lot more sense, neither company is big enough to bully/buy out the other, they'd be forced to collaborate as equals and whereas MS/Sony have nothing but overlap in targeting the same exact audiences, Sony + Nintendo actually are the perfect ying/yang to each other (ie: Mario brings in one audience, Uncharted another, Zelda brings in one audience, Gran Turismo another). Now that Kaz Hirai is president of Sony (a game guy) he can talk shop directly with Mr. Iwata as they're both Japanese video game veterans.
They'd also consolidate the Japanese development community as a whole under one banner (see: Monster Hunter, Dragon Quest, which Nintendo seems to have cut Sony off from, and MGS and Final Fantasy on the Sony side).