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.

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.

clearly the real world. There's simply no way, based on the laws established by the film, that he gets back to the plane in anyway other than reality.

The film failing to establish any type of timespan of how long its been since Mal died makes it tougher to guage, though one gets the sense its been a few years. If that was the case, IDK why his kids haven't aged at all.m...but I'd rather not bend my brain over that.

He finds Saito, even though in his mind it took forever, and wakes up right as he should: on the plane.

The totem, which while not falling, was clearly not perfectly spinning either.

Is Don's home in Paris or Los Angeles or are we unsure as to whether he exited the plane and went with Michael Caine back to some other city on a different plane? It doesn't make much sense that Caine would be raising Dom's children in Los Angeles yet be a professor in Paris.

Do the two scenes with old Saito actually differ?

Is being an architect or forger a special personal skill, or could there be specific drugs for being one?

Possible plothole: The flight took 10 hours, but the first dream took less than 12 hours (meaning that's just 60 minutes at most in real time if 1 hour dream time = 5 mins. real time) yet they woke up just 20 minutes before landing.

Or could the dream time got faster in the mission because they used a different drug?

Do the two scenes with old Saito actually differ?

Is being an architect or forger a special personal skill, or could there be specific drugs for being one?

Possible plothole: The flight took 10 hours, but the first dream took less than 12 hours (meaning that's just 60 minutes at most in real time if 1 hour dream time = 5 mins. real time) yet they woke up just 20 minutes before landing.

Or could the dream time got faster in the mission because they used a different drug?

If that is the case though, he would be the architect. The projections would have to come from the subject, but there is none.

In the bar where Cobb meets Fischer, at one point, he does draw the attention of everyone in the room. And he definitely wasn't the architect for that dream. And yet, at the same time, Ariadne and Arthur drew the attention of the people in the dream until they kissed.

Not a plot hole. COBB woke up 20 minutes before landing. It was never shown how long he was in the 4th dream. Could have been years. If anything you should ask, HOW did he wake up?

Everyone was staring at him in the airport? I know he had an Ocean's-Eleven-esque series of knowing glances with his team, but I don't remember -every- traveler stopping and staring intently at Cobb.