Wow very nice post original poster.

Are there shots of his hand to see if he's wearing it or not? Especially when he wakes up at the end.

I don't see it when he lets go of the top.

Was it always on the right hand?

If it was that hand, he did draw a lot of attention to it like "look at me"

Well, when they're on the plane it's not on the other hand:

http://www.imdb.com/media/rm901743360/tt1375666

But that's before they go into Cillian's mind.

However, the fact that everyone wakes up on the plane suggests they do get back to reality.

He wears the ring on his left ring finger. And no, it's not there when he wake up on the plane.

Anytime Cobb was in the frame (most of the movie) I kept a steady eye on his left hand. Even from a distance, like a medium-shot or long shot, you can seem the gleam of the ring when he has it on. Likewise, you can see that it's not there in the "real world."

Trust me, go back and watch the film again. It takes a very keen eye but I've watched it three times already and I haven't found any evidence to go against it.

Furthermore, you have to think of this as a filmmaker (which I am) and not just a fan. As a filmmaker you use certain visuals as reference points of character, time, emotion, etc. This is the purpose of the ring; as stated in my theory.

Again, it takes a very keen eye to track that hand the entire movie and like I said you can definitely tell when he's wearing the ring at a distance and when he's not. The gleam is unmistakable.

How can you tell there's no ring in the final shot?

You have to look at him when he enters that setting. Not just when he spins the top but when the scene starts.

Furthermore, the fact that he doesn't have it when he wakes up in the plane, or at the airpoint, makes it hard to believe that the next scene with him at the house with the kids -- that he'd be back in a dream. I suppose, it's entirely possible.

But again, you have to watch the entire scene and not just the spinning top segment of that scene.

Since the ring serves as a symbol for his guilt/attachment with his wife, couldn't it be possible that he is dreaming the last sequence of the movie? Since he is "free", could he be dreaming (on the plane, after successfully returning to reality with Saito) of being with his children and thus also not wearing his ring anymore?

The problem with the whole "it's just a dream theory" is it undercuts every emotional beat in the finale: the smiles at the airport, the reconciliation with Saito, us finally seeing the kids faces, etc...the ring missing and the sound of the top hitting the table after the cut to black are small details that just support the overall emotional arc of the film, which IMO is one of the greatest arcs in modern times.