Having seen the movie multiple times and having some rather in depth discussions with those who I've seen it with, I'm going to go with: There is no straight answer to whether or not the end was in reality or a dream, and Nolan wanted it that way. In this thread we've been touching on various aspects of the scenes that make us believe it might be one or the other, but there is never any real hard evidence. For example, you'll notice that Cobb only wears his wedding ring when he in in a dream. In reality, he doesn't. Now in the final scene, look for a clear shot of Cobb's left hand. There isn't one.
His kids turned around, and he could see their faces. I think that alone proved to him that it was real
It actually proves very little. Throughout the movie, Cobb refuses to look at their faces because he knows they aren't real. Now, remember what Mal says in the house when she's holding Fischer hostage at the end? When she says it's not about what you know, but what you believe? And in the same scene, Cobb finally lets go of his guilt, and lets go of Mal. Without the guilt, in his mind he is free to see his children again. The "charges" have been lifted. So all it takes is for him to believe that he is in reality and back home for him to be able to see his kids.
We can tell that he no longer cares about whether or not he knows what is real and what is not when he neglects to watch the top to see whether or not it falls. I mean, this is a man who was sitting with gun in hand ready to shoot himself if the top never stopped spinning in the first half of the film...and now he's content to just walk away?
In short, I'm inclined to think that he was indeed still dreaming and in limbo...but the real answer is that there is no answer, and that is the brilliance of the film. We're given two distinct possibilities but no definitive proof in favor of one or the other.
An another note, I notied some other folkes were confused as to how the top came in to play with Cobb planting the idea into Mal's mind. Basically, the top was a symbolic representation of her knowing what is real and what isn't, and she locked it away, out of reach. Since the top is used to prove to her that she's dreaming, locking it away effectively prevents her from reminding herself. So, when Cobb breaks in and makes it spin, he reintroduced the idea that her world is not real into her mind...but of course it carried on into reality.
That's really what the whole theme of the film was; the difference between believing something to be true versus knowing...sometimes you don't need to know as long as you believe.