So I have seen the movie twice, and it is easily one of my new favorites. Why? Because to have true understanding of the film, one has to ask the question "why does ____ matter?"
I love this question. It drives us deep into the roots and helps to bring perceptions and premises to the forefront of a discussion. It is the question that also helps refocus discussion and prevents us from chasing our tails in a discussion.
*** Spoiler Alarm ***
If you haven't seen Inception, I will be giving away spoilers, so if movies can be ruined for you by knowing what's coming, you'd be better off waiting till after the movie before reading this.
*** Last Call For Those Not Wanting Movie Ruined, Please Stop Reading. ***
Ok, meat and potatoes first. The central question that surrounds the ending of Inception is "when Cobb gets back home with his kids, is he in reality or a dream?" This question is at the surface important because we just got through a long quest for Cobb to get back to his kids. Part of this involved him confronting the doppelganger of his wife that he is keeping alive in his subconscious, that conversation involving heavily the topics of reality, perception, and experience.
But something key - something I think many are missing - is that in that conversation, Cobb isn't giving up the idea of using alternative realities to serve purposes. He is only letting go of the doppelganger he created of his wife. Why does this distinction matter?
If we go back the the central question and understand the nature of it, the reason becomes clear. The two choices we are presented with are assuming that Cobb will react differently when realizing he is in one world or the other. The assumption being that if he realizes he is in a dream, then he'd "move up" till he could be in reality and then deal with the circumstances there. If he's in reality now, he can stay and be happy.
My contention, however, is that we are packing more into the last scenes than can allow for either scenario to be confirmed...there just isn't enough presented regarding the realty vs. dream to comment at all, nor is there enough of the "post-Mal" Cobb to determine his long-term motivations and actions.
All that we do know is that while in the middle of performing a habitual reality check, Cobb is more interested in just seeing his kids than verifying he is in reality. This does not mean he will never be interested again, nor do we have any indication that it would matter to him if he was still in a dream state or in reality. All it means is that at that moment, after having confronted the doppelganger and having the opportunity to be with his kids, he was more interested in seeing them than verifying reality at that exact moment.
This is important because it means that he is willing to accept that experiences which are true and happy are worthwhile even if they aren't real. The doppelganger wasn't real or true, and this was evidenced by the fact that she couldn't remember things that she would have remembered if she were the real Mal. What Cobb was saying wasn't that he didn't want to spend time in limbo..he just didn't want to spend it with a shadow of the real woman he loved...pretending it was real.
At the same time, he is now in a reality where his kids are and he can be with them. So far as he knows and believes, this is the "real" reality anyway. This also helps explain why he was so quick to dismiss the top and go see his kids. He knows or wants this to be reality, so if even for a short while, it isn't going to hurt him to reconnect with his kids.
So, within the scope of the movie, it becomes irrelevant whether or not he is in a dream at the end, because for Cobb at that moment, it is irrelevant whether or not he is in a dream. Only when it matters to him that he is or isn't in a dream world should it matter to the viewer - and within the scope of the first film one cannot make a solid case for either perspective.