But now come the spoilery thoughts:
We're at the point where we need a name for a particular kind of ending, and I'm going to call it a Sopranos Dilemma. Let me first say what a Sopranos Dilemma is not: it is not a mystery ending that can be answered conclusively with the clues in the work. If you're left with an is-it-or-isn't-it question but, on closer review or with the help of a clue in that final scene, the evidence is firmly on one side of the equation, then it's just a riddle.
The Sopranos Dilemma is when you're left with a question, but the evidence on either side is inconclusive and in equipoise, and the movie (or book or show) is over, so you're never going to get any more evidence. The Sopranos Dilemma is an intentional act. As frustrating as it is to viewers or readers who want to believe themselves to be fictional detectives on the trail of A or B, "none of the above" is an equally valid literary choice. David Chase used it to comment on the relationship between authorship, fiction, and fandom.
The trick, though, is figuring out whether a particular ending is or isn't a Sopranos Dilemma. In Inception, of course, we end with an ambiguity. The top wobbles, but we don't see if it falls or rights itself, and therefore we don't know if Cobb is in a dream. The question is whether it's a mystery that can be solved with the information that Nolan presented to us, or whether we're just left to take, as Cobb and Saito say, a leap of faith.
I could be convinced that this is a Sopranos Dilemma, but right now I'm leaning toward mystery. The evidence on the side of it being a dream:
- We start, confusingly, almost mid-plot, just the way that one would start a dream (as Cobb reminds us). The movie itself is dream-like, with dream-like time and plot leaps. It could be, from beginning to end, a Cobb dream.
- The unreal quality of the real world. Why would Moll jump out of the window across from the room she and Cobb had rented? The room from which she jumped was an exact mirror image of the room out of which Cobb was leaning -- same flower arrangement, same couch.
- The trains. Remember at the beginning, when the original team is trying to get into Saito's secrets on the train? As the team disbands, the train passes a different train going the other way. I'd have to watch again, but I'd bet it's the same train that shows up on the street in LA and in the Cobb-Moll dream.
- Cobb's statement that he can't be an architect because "Moll won't let me." If what we see as the real world is a dream in which Moll is the architect -- and Moll gets to make the rules the way Ariadne did in Cobb's practice dream -- that could explain this line.
- The mirrors. Same practice dream, Ariadne closes the mirrors under the elevated highway, and Cobb and Ariadne are reflected into the infinite distance (great shot, by the way -- the absence of the camera in the shot recalls that Toulose-Lautrec painting of the bar girl with the mirror behind her, where the viewer or painter is missing in the reflection), suggesting that Cobb is already in an infinitely telescoping dream-within-a-dream-within-etc.
- The totems. Cobb uses his totem to figure out if he's in the real world or not. But it was Moll's totem first. You're not supposed to let anybody touch your totem because they could figure out how to manipulate it. If Moll were the architect of a dream in which Cobb continued to be stuck, he could not rely on it.