KSK does Inception. (spoiler warning)

"Ariadne: What’ll we do? I could try all that fancy building shifting stuff I pulled out when my character was first introduced and then never used again. On second thought… no."

(SPOILER)If I had to pick one, I think the ending was probably a dream. It just makes the most sense given what the movie says. The "entire movie was a dream" theory is just too big of a stretch; though technically possible, it just rests exclusively on loads of assumptions. Someone mentioned before that Cobb always has his wedding ring on when he's dreaming, which is an excellent catch and a very persuasive piece of evidence for the ending being a dream.