Colossus is in the "Last Defenders" title after Messiah Complex. He leaves the X-Men because of something that happens in Messiah Complex. My best bet, even though I don't like it at all, is that Shadowcat is killed or goes Skrull or both. Colossus has always been my favorite X-Man and to have him come back only to lose the one he loves just sucks. Would be a great story and give him a tragic aspect to him, but it still sucks.

Yep. The solicits show Colossus leaving after MC is over, and the only thing that could drive him to do that is Kitty's death. So my money's on her.

It's also fitting with Whedon's run. Colossus and Kitty have FINALLY hooked up and become a happy couple, and those don't exist in the Whedonverse. As soon as two people become a couple, one of them either turns evil or dies. Whedon's not going to be writing X-Men anymore, but his writing influence lives on.