I think the entire movie took place in Limbo; Cobb's to be exact and Ariadne was sent in to his dreamscape to bring him out. There were clues throughout the movie, given by support characters. (Remember the "Opium Den" scene? The old man there said to Cobb, "They are here to wake up. You of all people should know." that set off alarms in my brain) The biggest clue is the Totem. Cobb doesn't use his own, but his wife's. Each extractor/architect character emphasized the importance of having your own totem that no one else may touch. It grounds you in reality. I think the stuff with his wife did actually happen, but his grief and guilt trapped him in his own Limbo, where he made an elaborate projection that allowed him to let go of his wife's memories. That's why the top was still spinning at the end.

Of course, this is all just a theory. It just might be that everything that happened up to waking up in the plane did happen in RL, but that Cobb never woke up, and his getting through US customs and reuniting with his children were projections.

Your thoughts? IDEAS? Plant em on me!