After watching Inception for a second time this weekend I can say with certainty that I know which world Cobb is in.
It should go without saying that this discussion will invoke spoilers so people who have not seen Inception (and honestly how many people could that be?) tread carefully.
Armed with a secondary knowledge of all the tricks to distinguish between Cobb's (Leonardo DiCaprio) dreams and the waking life my girlfriend and I spent our Saturday deciphering every little moment in Christopher Nolan's latest mind-bender. For every dream Cobb is in his wedding band is placed securely on his ring finger, when he is not it disappears. A key point to remember during the last scene of the film. As Cobb sets down his luggage to spin his totem one last time his band does not appear on his hand. More importantly, the children are wearing different clothes than the same outfits during all the previous scenes.
Whether Ariadne (Ellen Page) is a plant by Miles (Michael Caine) to convince Cobb to "come back to reality" or just a concerned bystander is for you to decide. Personally, the Saito-Fischer substory seems too real for it to be a facade to lull Cobb back to the real world.
Now, let's address those questions surrounding how Cobb got back from his dream to limbo to real life. The first is easiest to explain, after Mal (Marion Cotillard) dies Cobb presumably dives off the crumbling building in the same fashion as Fischer (Cillian Murphy) and Ariadne where he is transportated to Limbo. Not long afterward Cobb washes ashore and is taken to Saito (Ken Watanabe). Gun in hand Saito and Cobb help each other recognize why they are there and what Cobb has to do to get back. The last shot is of Saito picking up Cobb's gun before we jump back to the airline. Christopher Nolan could have been more clear in lengthening the scenes before the cut-aways, but the point of Inception was to always leave a little doubt in your mind.
The main criticisms levied at Inception is that it is flashy, yet below the surface, entirely devoid of substance. Now, that I just will not believe. There are Thoreau-ian themes throughout the film, "To be awake is to be alive." Miles (Michael Caine) mentions as much to Cobb when he begs him to, "Come back to reality." Cobb has been away from his children for, what it presumed to be, a few years running heists that won't bring him any closer back to his old life. Only in awakening can Cobb truly live his life in a "perpetual morning," which is to say that he will never experience the darkness again.