sure that the plane is reality? what about cobb's spinning top at the end? after the come back from limbo, cobb and mal could be still dreaming, that's because he is unable to dream - he is in a dream. (?) or maybe the plane was reality, and he does not come back, at the end he is still dreaming?
The spinning top actually isn't Cobb's totem, it's Mal's. Cobb's totem is his wedding ring and when he's in a dream his wedding ring is on because to him they are still together there, when he's in reality it is off. In the end of the movie Cobb's wedding ring is off so in the end he's in reality