Inception certainly appears to released us all from the post-Lost limbo, and we can essentially drop all the hypotheses currently in circulation into one of three boxes: It's a dream, it's not a dream, it's inconclusive...either because Christopher Nolan intended it that way, or because, like the makers of Lost, he ended up thinking that the best way to harness hidden depth was to just let go a bit.
After a single viewing I suppose I'm leaning towards the intentionally-inconclusive camp, for I believe that in order to be faithful to the material — our greatest metaphysical enigma — Nolan simply had to deliver an enigmatic ending. There was just no other way for this to end.
The clearest evidence for the concluding scene of Inception not being set within another dreamworld is Cobb's wedding ring. Whenever we apparently know it's a dream, he's wearing it and when he's apparently back in reality, he's not. As such it must be his totem, and he's clearly not wearing it in the final scene with the children. Yet I don't find this conclusive. The whole idea of totems could itself be an inception. The last shot of the movie is the spinning top, but that is Mal's totem not Cobb's.
Some have pointed out that the children in the last scene are exactly as Cobb imagined them in his dream. Not so, says the movie's costume designer, who remembers creating separate outfits for the two versions of kids in the garden. Hmmm. Nobody seems to have mentioned all those knowing smiles Cobb gets from his co-conspirators at the end. You'd have thought they'd have immediately got together for a chatty debrief, rather than floating out of the airport as if they'd all just willfully exceeded their daily dose of soma.
And then Cobb spins the top and walks off as if the issue isn't really all that important to him any more.