Two meta-comments: first, this post contains something (many somethings, actually) that may spoil the ending, so don't read it if you don't want to do that.
Second, I welcome anyone's comments-- but I want to give a special call-out to Jason Kennedy on this one.
So, here's my theory for the ending (spoilers coming now-- avert your eyes!):
I think the key to it is that Cobb's totem switched. Somewhere in the middle of the movie the switch began: I think it was when Cobb and Ariadne were watching his children play outside. The switch solidified toward the end, before he woke up-- actually, it was about half-way through his last time in limbo, when he finally let go of Mal.
Cobb's new totem was his children's faces-- NOT the tipping top. When he saw their faces, he knew it was real.
I know what the best objection to this theory is: others aren't supposed to know the totem, and he told Ariadne all about that moment with the children during the dream. But there are several things that overrule that objection:
- First, a lot of the totems were known, to a certain degree. Cobb actually told Ariadne what the key to his spinning top totem was, and Arthur revealed the key to his totem too. There's no inconsistency with the storyline for Ariadne to know that the key to knowing reality would be for him to see the children's faces. It was the faces themselves-- and what they looked like-- that was the totem.
- Cobb's totem was very personal-- the top was actually Mal's totem before it was his, wasn't it? So, for him to let go of her means that he would need a new totem; it makes sense for the kids to be it, since he chooses highly-personal totems.
- No one else would know what the children's faces looked like, except Cobb's father, who (we get the picture) long-ago left behind the work of dealing in dreams. So the faces would be a perfect totem.
- The kids in the movie were different in the end (reality) from the ones in the dream. It was subtle, but they had different actors playing two-year-older children. Had Cobb been in a dream, they would have remained the same, wouldn't they?