I've been wanting someone to talk to about this movie for days!

My theory is that Mall was right, and Cobb was still trapped in another dream level. I think she sent Ariadne to try and get Cobb out, to put an idea in his mind that he had to let go of his guilt and that would help him get out of the dream. I think this is why he was using Mall's touch-stone for reality rather than ever showing us his own (and why Ariadne's was a pawn). And this is why I think he was still dreaming at the end: because I don't think Mall is dead, and I don't think there were ever any children.

Of course, I could be wrong....