Not XBox. Microsoft sucks for that. PC/Mac and PS3 players can do cross platform co-op.
As well.. I've finally finished the game. It's kinda short (not to the point where I hate how short it is though, and not dragged on forever). I love the humour and it had a lot of good moments. The puzzles weren't impossibly difficult, but none were super easy.
Ultimately, I think the best game I've played since the original Portal.