That scene is all about whether Cobb left limbo with Saito or whether he listened to his subconscious and created his own "reality".
I know I'm exceedingly late to the party, but I disagree with this and say that the ambiguity of the final scene wasn't meant to provoke a "dream v. reality" debate, but to say, simply: It doesn't matter
whether or not the final scene is dream or reality. What matters is its reality to Cobb. He walks away from the "test" because he's already accepted what he sees as reality. He's taken a leap of faith (i.e. no tangible test is necessary to him to either prove or disprove his belief). Or to put it more simply, the point of the film is its journey (Cobb's ability to forgive himself and let go of his guilt), rather than its destination. We're not shown the top falling (or forever spinning) because it just doesn't matter.
I say this because *if* the final scene is intended to spark debate amongst moviegoers as to whether or not the final scene was real or a dream, then I'd say Nolan was being little more than an amateur hack -- conjuring up a cinematic "trick" by not taking a stand in his own film. If the whole point was some arbitrary ambiguity, then the "cut to black" before confirming one possibility over another would be ridiculously cliché (and cowardly) in this case, and far more problematic than the ending of, say, The Matrix Revolutions
(which, at the very least, had a resolution).
But I don't
think Nolan is a hack (Inception
, like so many of his films, is, in fact, pretty good) so therefore I do
think the debate about about the final scene is, ultimately, a moot point as far as the film itself is concerned (as an intellectual exercise it's fun to debate and, for the record, I believe the entire film, including the final scene, is a dream). The fact is, we do get a specific resolution, which comes from the fact that Cobb is reunited with his children in a moment that is as real to him as "reality" is to the rest of us. It's real enough that, if he is dreaming, he won't wake up.
My criticism with Inception
is that, like the lower layers of dreams, Cobb's story is simply buried too deeply under exposition and action set pieces. The result is that his own journey is minimized -- as is the focus on themes such as the nature of reality, redemption, forgiveness, faith, family, and what it means to spend a lifetime -- be it fifteen minutes or fifty years -- with someone you love.
Ultimately, I give the film a 'B' for its exceptional creativity, craft and performances. But it misses being truly exceptional because it felt too overbloated with rules and 'splosions (really, the editing of the final climactic scenes, in which we're repeatedly shown a van being shot at or falling off a bridge, or a zero-g elevator rescue, or unidentifiable snowpeople shooting at one another were all truly redundant). The miscues made it more difficult to truly engage with Cobb's conflict -- which was the ultimate point of the story.