I need to see it again, but my gut reaction to the last shot was "it won't stop spinning." I was content thinking the whole movie had been an elaborate dream that Cobb was having, and that Mal (his dead wife) had been right all along. He needed to wake up.
But after contemplating further, it felt like there was too much back-stepping justification to truly commit to that theory. For example: isn't it strange that we only see one small part of Cobb's house, both in "dream" and "reality?" And that when he does finally see his kids, they're once again in the backyard, in very similar clothes and positions? And can we really trust the logic of totems, when it's a concept he'd altered in order to "incept" Mal about the slipperiness of reality?
Regardless of all these questions, I feel like if director Chris Nolan wanted me to land hard on a specific interpretation, he would have given it. Judging from his past films, namely Momento and the Prestige, Nolan tends to reveal the true twist. Leaving the ending of Inception so open has more to do with broader notions of the film's themes, and less about making an either/or decision.
Cobb's dismissal of the totem should be more important than if it truly stops or not. Cobb finally deals with his horrible guilt, by confronting the shadowy version of Mal and making the choice to move on. His perception of himself and his situation changes, much like Fischer's own relationship with his father. In each case, their own personal reality has been altered. It's as if Cobb "incepts" himself.
That creepy old man in the strange opium/dream basement sums it up nicely: "Their dream has become their reality. Who are we to say otherwise?" We each have our own reality, and we just witnessed Cobb finally committing to his own.
If you want to read this theory as explained by a person much smarter than myself, I highly suggest this post. Here's a quote that I dig:
In the end, in absolving himself of the guilt, Cobb realizes it’s rectifying your inner reality, not the outer one, which is most important. With this movie we get bogged down by questions of what is and what isn’t concrete reality and that is all beside the point. It’s only the reality of one’s own mind that matters. The reality you believe to be is the only reality that exists.
With that in mind, wouldn't it have been terrible if we saw the totem fall?