Anybody else think of the book Godel Escher Bach when they were watching this movie? Beautiful how the movie wraps in on itself like a mobius strip.
Don't read if you haven't seen it yet!
Saito dies at some level (if I remember right at the top dream level) and he enters limbo. Cobb goes there from the snow compound dream level but Saito has already been there 20-40 years (maybe they said in the movie, I forgot that too). Cobb then helps him remember Saito is in a dream so they can wake up together from the limbo state.
Except they don't, you never see them kill themselves (to exit the dream) or pass through the other levels of dreams, it just cuts straight to the plane. Then at the end he sees his kids, but they're exactly as they appear in his dreams. The top continuing to spin just makes it more explicit.
So he never made it out of limbo, when you see him awaking in the plane that's just him creating his own reality inside limbo, one where he gets to go back to his kids.
Nolan was just doing that to mess with the audience. Yes, the kids were wearing the same clothes, but the top started wobbling right before it cut away. So the audience is left not knowing if he is stuck in limbo or not. It could be interpreted either way.
Anyways, an easy way to tell if you are in a dream without relying on a totem (in the movie's universe) is to look at an analog clock. The numbers appear upside down in the dream world. For some reason this was only illustrated once in the movie.
Indeed, I remember from my own dreams that text and numbers behaves oddly (disappears/changes when you look around).
I've also heard this technique is used by lucid dreamers (http://en.wikipedia.org/wiki/Lucid_dream) to figure out that they're dreaming so they can begin to take control of the dream.