Mal is still alive and was right (Inception theory of mine !)

I believe the entire movie is in Cobb's head. The 'real world' is actually Level 1 of Cobb's.
Cobb's father, Michael Cain, exists in the real world and hires Saito to bring him back.

Real World = Michael Cain's scene time
Level 1 = Cobb's 'real world.'
Level 2 = Cobb's Level 3
Level 3 = Cobb's Level 4
Level 4 = Cobb's Limbo