It's a very good movie. I see why people have been raving about it. There's a lot of room to theorize at the end of it, and so, I shall.
We all walked out of it pretty much in agreement that the fact that the top is spinning steadily at the end of the movie is a sign that Cobb is still dreaming. So what happened?
My theory is that the entire movie, start to finish, happens within a dream. "Reality" is at least one layer down, and Moll (Cobb's wife) was, in fact, correct about still being asleep.
The entire thing is an inception being performed on Cobb, and the idea that they're trying to implant is that he's still dreaming and needs to wake up. Probably all of the speaking characters (possibly even Moll?) are members of the inception team, but the leaders are Saito and Ariadne. (Note the dead-giveaway nature of her name, by the way: Ariadne led Theseus out of the labyrinth.)
After they ride the kick all the way up through the layers to the top, and awaken on the airplane, then there was supposed to be some turbulence that would be the final kick that takes them out to the top. But Cobb gets lost down at the bottom, where Saito is trying to remind him of something that he has forgotten -- that he's still dreaming -- and instead of riding the kick upwards, just goes into a fantasy of arrival and reunion with his kids.
I'm not sure whether Moll is participating in the dream or whether she really is just a subconscious projection of Cobb's. Given the behavior of the top, it may in fact be her dream that he's lost in at the top level.
I'd love to hear what other people think!