The top does not straighten out again.
The kind of evidence for saying the final sequence is a dream is the same kind of evidence for saying that we never, ever, see reality during the entire movie. The mazelike streets, the nightmarish squeeze, the inanity of corporate gunsels roaming the streets shooting, the sudden appearance of help. True, it breaks the rule that dreamers start in the middle of a dream, but the top falling says that the kids are real too. The problem of course is that if the whole movie is a dream, huge numbers of scenes are pointless.
Well only a small handful of people are suggesting that the entire movie is a dream. I think most of those arguing for a dream ending, like me, simply think Cobb is the one still dreaming at the end.
Having the whole movie be a dream truly WOULD cheapen the entire experience, and I can't see Nolan wanting to do that kind of thing (even though it's certainly plausible given the way he tells his story).
For me what it comes down to is that the ending with the kids just seems a little TOO sweet and perfect. I certainly want Cobb to have that happy ending, but the way it comes about-- with the smooth, dreamlike transitions from the plane to the airport to the home-- just seemed too damn easy given all he had been through.
I'm not arguing that it's SUPPOSED to be a dream; just that Nolan wanted us to walk out not knowing for sure either way. To suggest that the ending was just a quick "fake-out" before telling us Cobb really made it back is not what I think he was going for (and nothing I see any definitive evidence for in any case).