Apple, most likely. They've already expressed interest in creating a console, and are getting into the hand-held market at the least with the iPhone and iPad. They have the capital, interest, and commercial motivation to step in.
I know that some seem to think Sony falling through isn't going to happen, but you have to remember: We aren't talking about Sony as a company collapsing here. We're talking about shutting down their gaming division. Sony does a lot of other stuff, and with PS3 sales being as lackluster as they are, the last few PSPs bombing like the Fat Man, Move being a commercial wash-out, and now the very real chance that they are going to lose billions on a security breach that they didn't think to prevent, I can see the Board of Directors at Sony pretty much going, "Well, F*** this guys, let's do something else," and moving on.