(SPOILER)The problem in believing that the end was a dream is that it is not supported anywhere in the narrative. We can know that the reality he returned to is reality because we have seen his totem topple in it before. Unless the transition from reality to another dream (for which Cobb requires the use of the machine) happens off screen, everything adds up to the final scene taking place in reality. One could suppose that such a transition from reality to dream occurs off screen, but I won't accuse Nolan of such shoddy storytelling. One might just assume that the whole movie was actually a dream and that we were told about his totem wasn't necessarily true, but that sort of distrust for an entire narrative leaves the story meaningless anyway.