I caught this on the first viewing, but it took me until last night to get a chance to check it for accuracy.
A lot of people are speculating on what is real and what isn't, what is in Sam's head and what isn't, etc. I'd like to pose another theory that I've seen hinted at once but not yet expanded upon. Not that I believe it, I just like this game. Sam said yes to Lucifer, yes? Sam is Lucifer's vessel, yes yes? There is no getting out of that, right?
But ... Lucifer is still in the cage, that much we know for sure.
"I needed a threat. And we need to keep Michael in Lucifer's cage." - Castiel
... or do we? O.o