i know old thread is old, but i watched this again yesterday after getting it and a sweet blu-ray player for christmas
anyways...i'm pretty well convinced now that cobb was still dreaming at the end when he got back to his kids, and it all goes back to the totem, and the fact that his originally belonged to his wife. earlier in the film arthur explains to the architect chick that she can't touch his totem because it would defeat the purpose of it, which means that cobb's totem is "tainted" by his wife...the top might still fall down when he spins it, but it's because his subconscious projection of his wife is knocking it down.