<span style="font: 16px Candara, Skia, Corbel, 'Trebuchet MS', Georgia, Geneva, Helvetica, Lucida, 'Lucida Grande', Arial, sans-serif; color: #38bcee;">I'd like to point out that (SPOILER)it's also entirely possible that the entire thing was a dream and Ariadne, Seito, and all the other characters are projections created by Cobb's mind to convince himself that he's in the real world. This seems less likely, but it's still possible.

In any case, the only way we know the top falling would be indicative of the real world is because Cobb said so. How does he know this? He never specifies. It could be entirely possible that he just believes the top falling would be indicative of the real world.

At any rate, I don't think we're supposed to know for certain whether the ending is a dream or not. It's pretty clearly intended to be ambiguous. The multitude of ways in which the film can be interpreted is one of the reasons I liked it so much.

And yes, the ending is definitely Nolan's inception on the audience.