^ Actually the more I think about the reasons put forth by everybody, the more I think it *must* be reality. And then I get more confused as to why Nolan would choose to end on the reality/dream question-mark. It makes it nothing more than an affectation rather than any genuine conundrum.
And therefore I go back to the there-must-be-more-reasons-for-it-to-be-a-dream way of thinking... But what? Except that he deliberately sets that up.
I think he puts the doubt in there just for the question mark, just to start discussion. Look at this very forum, three, four threads on this movie and in them discussion on whether or not people thought the ending was in "reality" or still in the "dream"? We couldn't have this disucssion if the ending was obvious and clear-cut. That's great film-making, something as simple as the white-flash at the end of "Total Recall" is the movie the "real world" or was it the memory implant Quaid had paid for?
Actually Total Recall is better in that regards. You can never know one way or the other whether he truly had that adventure or it was the excellent adventure-vacation from Total Recall playing out forever and even in his mind. (Unless you get told by the creator/writer - kinda like what Ridley Scott did with Bladerunner)
In this case, I don't think there's any doubt in *your* mind, (once you've thought it thru) that this is reality.
Can you actually formulate a good reason for it being a dream and "make it stick"? I *see* the evidence of it being a dream/limbo there - children not really changing, Caine at the airport, us not seeing how Cobb woke up thru the levels, all the way down to the top which spins for an inordinately long time and seems to "fix" it's wobble about 3 times (from what I remember from that one-time viewing). But what are the *reasons* for it being a dream? All of them are unsatisfactory. Either Cobb is still in limbo or he has returned to the "real" level from the movie - but now that level is a dream. Why is that? All of it is "unreasonably unfathomable" (in short, is there a theory which can explain what is happening without being "it-just-is-a-dream" at some level?)