I think he was definitely in the real world at the end, he said the top spins perfectly without any wobble in the dream world and the top was noticeably changing speed and wobbling.

Why didn't Mal just use her top when she got out of the dream to prove she was in the real world. I mean she couldn't physically take that piece away from her while in limbo.

I'm guessing the last few seconds that Cobb was in a dream because for one his children would be a bit older right? From the sound of it, Cobb's been on the run for a while so they'd have to be older. Though also he said in the movie he never saw where his kids turn around before he leaves. Why would he suddenly see that now though? I don't know very odd way to end it but I like that it's keeping me thinking.

The "totem" will fall over. The last scenes are real. I am enjoying the various theories, but I haven't found one so convincing as to change my beliefs.

When Arthur is explaining to Ariadne (nice Greek Mythology reference, by the way) the concept of a "totem", he said something about her touching his would defeat purpose of it, right? Yet, in the limbo it's Saito who spins Dom's totem. Hmm...

If he is in limbo, what happened to his body on the plane? Sent to a prison hospital for the rest of his days? A private facility owned by Saito?

there's absolutely no definite answer (no matter what theories anyone comes up with). The moment I thought was absolutely priceless and utterly delightful was the little hiccup the dreidel makes, and juuust as you're about to see whether it completely loses velocity or keeps artificially spinning...... CUT TO BLACK. Best intentional tease of audience expectations since the end of PATRIOT GAMES.

I also think he made it out of the dream. They were on the airplane. He saw the kids faces. He met his father.

THough the kids probably should have been alittle older and different...but time moves slower in real time

I'm almost sure the whole movie is a dream (and we're stuck in it lol). I've been going over the scenes that supposedly take place in awake world and there's a lot of surrealism there too. The thing that keeps bugging me is that no one but Cobb had a motive to ever participate in this elaborate inception. Are we to believe the people in his team were willing to risk their lives to help out some guy take out his competition?

The bigger clue, btw, that he is lost in the dreamworld at the end is that when he/we see his kids at the end, it is just an extended scene of what his been envisioning throughout the movie. A return to his life would surely have produced a different visual.

the plane is just another level of dream so he returned to that one but not fully to wherever reality is...

Well as Cobb says in the beginning... you never remember how you got to where you are in a dream. The plane was a perfect spot for him to start his new dream.

Before I get started, let me make it aware that I consider Nolan to be a master of deception, which is gonna be the basis for a lot of my conclusions

First off, I don't think it's worth much more than entertaining the notion that the entire film is a dream. It's an idea that is virtually impossible to prove without Nolan outright coming out and saying it's so, and is ultimately counterproductive in my view.

Secondly, I've heard people say that Saito was actually the mastermind to achieve whatever his goals may be. He is certainly a shadowy character, and some have pointed to the scene in which Dom first uses the sedative. In this scene, Dom wakes up from the sedative, is shaken by it, and then spins his totem. It's significant here because the totem falls off the sink before we can judge if it continues it's rotation. In my view, this scene is here to act as both misdirection by Nolan, and as a reason for why Saito was able to look at the totem in Limbo and remember his promise to Dom.

The ending, of course, is going to be (or just is?) the biggest point of discussion for a lot of people. There's a definite sense of ambiguity to the ending, but I think it's absolutely reality for the following reasons:
1) Once they get off the airport, all of the members of the group and even Fischer acknowledge Dom in a way that none of the projections in any of the other realities did except, of course, for Mal. Mal is the exception because she was the part of the mind of Dom that wanted him to stay with her in Limbo forever. With him finally letting go of his guilt, it doesn't make sense for his mind to continue to try and trick him.
2) Some have brought up how his children are in exactly the same position and are the same age in which they were in his memory. Initially, I thought of this as perhaps a reason for why I was wrong, but after seeing it again, I realized a few things: Their position, while in the same area as his memory, is not identical. James and Phillipa have actually switched places. Also, they are indeed older than in the memory (something that is confirmed by the credits on IMDB actually, hehe). Additionally, some have postulated that Dom has been on the run far too long, though I do not believe to be true. In the phone call after the botched first job, it's evident they are still quite young, as James does not have a grasp on the concept of death. If you want to believe that is also a dream too, then I feel you're back in the field of counterproductivity.
3) The spinning of the totem is the big one for a lot of people. It's very evident that it's indeed slowing down and wobbling, but the cut to black leaves us with that nagging feeling. Some have said the amount of time the totem is spinning is far too long anyway, but I disagree, since the first time Dom spins it it remains for roughly the same amount of time. Also, the fact that Dom revealed how the totem works to Ariadne has led to some to believe that this makes the spinning a non-factor (those believing Ariadne actually performed inception on Dom), and that it's also not initially his. Since it's previous owner (Mal) is dead, that latter point is a not an issue. Also, Ariadne did not touch or hold the totem, so she could not possibly it's texture nor it's weight.

Regarding the ending, I've read all the theories here, and I'd say that most likely, he's in the real world. I think it was obviously losing speed, no matter how fast it cut-to-black right after that. However, the opinion Magnus posted...that'd be AWESOME if this was actually what happened. Which makes me want to believe that.

I think he's in a dream. There's some clues like the children's placing set up exactly like his memory, the top never spinning that long in the past. But the biggest one to me is the illogacy of the Leo, Saito meeting. Why would Saito be in Leo's limbo, wouldn't he be in his own? If he is in his own, why is Leo there? Why is Saito old and Leo young, both should age accordingly if they are in the same limbo. Why are Leo's eyes black? What exactly happens between Mal and ending up in that water? None of this is explained, we just get Leo in the water like in the middle of a dream. There doesn't have to be an explanation. I believe we - and Leo - jump from falling building sequence to water to Saito meeting like in a dream, and all of this is done so Leo can have the limbo he wants. The limbo where he doesn't know it's not real... where the truth is locked away in the safe, or in this case left on the table without being seen. It reminds me of the scene in the Sixth Sense (of all movies) where we learn Bruce never goes in the basement because he doesn't want to. I think Leo knew he'd never found Saito and didn't care to. It was a red herring to get Page away. He knew he wasn't coming back.