My Theory…which goes like this…which is my theory…and no one else’s…is stated like this:
From the point of the movie where Cobb (Leonardo DiCaprio) goes into the Yusuf’s (Dileep Rao) basement and enters the dreams of all the other dreamers down there, the rest of the movie is Cobb’s dream and Cobb never comes back into “reality”. At the end of the movie, he is still down there in the chemist’s basement with the rest of the dreamers.
I base this hypothesis on the fact that when Cobb “awakes” from the dream in Yusuf’s basement, Cobb spins his totem, but he (and the audience) never sees if it topples because he is interrupted by Saito (Ken Watanabe).
I have no idea what this does to the rest of the movie’s story, but it does enhance the opinion of those who believe that in the final scene the totem does not topple.