SPOILERS ABOUT THE ENDING:
I think that it would have been better mindfuck if maybe the whole thing was his wife's dream. She is the only other person who would know the weight of Cobb's totem and therefore it would react correctly in the dream world.
One thing that makes me think the ending was a dream was that his kids look exactly the same as they have in his previous dreams (except he can see their faces). It is implied that he hasn't seen them in a good while and that they should be older. The fact that they still look exactly like he remembered them makes me lean towards it being a dream.
Yeppers. The ending's not very ambiguous, really. It's like the ending to the Prestige, in that regard--it's twisted, but it's not really debatable.