Nolan has created a movie which creates a great deal of ambiguity - whether by mistake, through story driven fast edits, or intentional dream state fast edits to allow the viewer to decide for themselves whether he has made it back to reality or not.
By the end of the move we can see that Cobb is no longer interested in his totem, he is convinced that all the decisions and actions he has made have lead him back to reality.
But we, as the viewers, now have the decision based on our interpretation of preceding events as to whether he has made it back. The first time I watched this movie I left the theatre feeling as though I was playing "catch-up" with the storys complexity, but having seen multiple viewings my personal take is that he is still in a dream state. The mombassa maze, the spinning top totem which both Ariadne and Saito know about and could recreate, the fact that Cobb wants to believe in the last reality also renders the spinning top completely worthless, hence, he ignores whether it continues to spin, or falls over.
Along with these points we have a number of scenes that occur, both in reality and the dream states, that further push the ambiguity:-
Early in the film, Dom Cobb's phone conversation with his son and daughter include ambiguous dialogue, where his son asks "is mommy with you?", to which Cobb responds "mommy's not here any more". His son responds by asking "Where?". This in itself is a strange question. A normal response would be "Where is she?". Asking just "Where?" tends to infer that the question is aimed at where in existence is she.
Nolan himself points out that one of the most important pieces of dialogue in the movie is when Mal suggests to Dom - "admit it, you don't believe in one reality anymore".
We have already seen proof of this multiple times - any moment where he sees his children, or hears breaking glass, or sees Mal, he momentarily loses his grip on reality. The most notable scene is in the third level dream where he has her in his sights but won't shoot, even Ariadne cannot convince him by this stage. He only takes action after he sees her shoot Fischer in an attempt ruin the plan and then drag Cobb into limbo to be with her.
Cobb also has a tendency to create dream memories, something he warns others NOT to do due to the fact that it is the quickest way to lose your grip on reality. He thinks he has it under control and wants to be able to fix these "regrets", but can he really do that in the real world? Or would it take a dream state to allow him to "regret nothing". As the "wise-old-geezer" in Mombassa states - "they come to be woken up, who are you to say otherwise". Any dream state can be created to allow a person to experience catharsis or happiness.
Another point that has already been mentioned:-
Cobb's son - As stated previously, the movie jumps around as much as your typical dream state might. Why does the film open with the limbo scene, where we first see Cobb's son building a sand castle on a cliff, to then switch to Cobb inside Saito's "house on the cliff", then to be shown all the events that lead us back to the same moment in limbo at the end, in the same "house on the cliff". We see Saito reach for the gun, but we are not shown whether this gun is capable of bringing them back to reality, or leaving them in a further limbo state, one where Cobb has re-created a dream life with his family. A clue to this is the residual memory that his son says in the very last scene whilst we, the viewers, are concentrating on his spinning top. The son says "look what I've been building, it's a house on a cliff"!
