WARNING : I DO NOT KNOW HOW TO MASK POSSIBLE SPOILERS.
Hi. I joined just to discuss this movie.
The audience gasped at the end of the screening I attended.
He had to be dreaming at the end, and IMO this was given away an hour previous. When Cobb's wife commits suicide, she's across the street sitting on a different building and Leo's saying "come step back inside" like she's just supposed to fly over to him ... and he doesn't need to shout for her to hear him across the street, and any investigator would know she didn't jump from the trashed room across the street from her. This was a clear and deliberate giveaway, I hope, since it could only make sense within a dream. If the director intended that all to make sense as a non-dream reality, then it's total garbage. So too several other things, but that was the most obvious.
At the end the director made sure to show the top spinning on and on despite almost toppling earlier when it hits a groove in the wooden tabletop. Plus no explanation how Cobb got out of that last dream, the fact that he was polluting other people's dreams with his subconscious which isn't supposed to happen right? His wife always shows up, plus when the girl architect tells him about the air vent shortcut into the snow fortress, suddenly the bad guys turn around and the Aussie guy says "they're turning your way, almost as if they know something."
These and other things happen because it's all Cobb dreaming and his wife was right about him never having gone back to the reality layer. Grandpa gives his best student to an obsessed criminal doing very dangerous things, etc. Too convenient. Ironically, Cobb was the one with the idea stuck in his head - that he was in the real world when he wasn't.
One other opinion - it seems to me the team can't be shot in the dreams, and they almost act as if they know it 'cuz in the chase sequences they'd have been Swiss cheese and didn't seem to try too hard not to get shot. Only the Japanese guy got shot 'cuz he doesn't know any better? Nothing else harms them - crashes, tsunamis, etc, so why would imaginary bullets? Or was this the usual movie BS where one cowboy shoots 10 indians with every bullet, but never gets shot themselves?
EDIT - why'd the wife put the totem in the safe? People are supposed to have their own totems, untouched by others. Was it cobb's all along, or did he take it from her? A top is hardly a "deep dark secret" ... unless it has something to do with Cobb existing in her dreams as a memory? That weirdness hasn't been touched on, and I doubt it would've gotten so much screen time if it were meaningless.