The patent system is fucked. I've seen less-watertight patents get upheld and some blatant rip-offs get off scott-free, ESPECIALLY when it comes to software patents.
Whatever state MS is in, there's no doubt that they are going to be a more aggresive comapny with Ballmer in charge. Sony better hope all the chairs are pinned down if they're ever in the same room as him.
Regardless though, it's up to Sony if they wanna try and up to MS if they wanna sue. If Sony do think MS will sue, they won't pull the trigger, especially as they've already lost over $3 billion on the PS3 already. They'll probably just ask themselves wether custom soundtracks would sell many more consoles and I don't think it would. Sure, it's a
nice feature, but right now I doubt anyones main reason for getting a 360 over a PS3 would be the fact that they could play their own music on it...
EDIT: Good god, I've just been on the forums of n4g to see it and from what I've read they seem to have been further back in the line when rationality was handed out than GameFAQs posters.
Then again, there's not really such a thing as a good video game forum these days, unless it's dedicated to some underground, specific topic.