INCEPTION'S ENDING*mega spoliers*

relevant: http://www.collegehumor.com/video:1939234

4. It's a sequel hook. If he's dreaming, they can do a new movie where the idea did/did not plant successfully and exploring what happens when he wakes up.

I'm pretty sure it's simply meant to be ambiguous, and that's the real reason the film cuts off before the top stops spinning (or fails to). Cobb simply doesn't care anymore, so neither should you. This is the reality he's sought the entire film and this is the reality he's content with.

Pretty much this. They knew that by leaving it ambiguous, it would not only spark discussion topics (such as this) and make the movie stand out from the other 87 action movies that came out this year. There's never definitive answers to discussion questions.