http://en.wikipedia.org/wiki/Inception#Ending - Nolan himself noted that "I choose to believe that Cobb gets back to his kids, because I have young kids. People who have kids definitely read it differently than those who don't". Good enough for me.
You're still right he meant to leave it up to the viewer, but it completely destroys the rest of the movie if everything is a dream. People want to play games with the rules the movie presented and hypothesize about the whether Mal is in the "level above"... but everything we think we know about the rules comes from that level. We learn about the multi-layer inception, the wife, the concept of limbo, everything in the movie, on that level. If it's all a dream, then there's no target to the obsession in the first place, no children, no wife, nothing.
Incidentally, note I'm sort of making a metapoint... if Nolan came out and said "Yes, it was all a dream" I would accept it. But it would still dramatically destroy the movie.
I'm also sort of hostile to the "all just a dream" idea, whereever it appears in fiction, because it's redundant. It's already just a dream, a movie, a book, a TV show, whatever. It's already not real. Saying that in the context of the not-real work of fiction the entire story was also not-real is silly. (Note the word "entire".) It started at the maximum level of not-realness from the very first word or frame. And it's a short trip from there to the Bergman/Braga incoherent style of ass-pull storytelling. (Or Tennant-era Doctor Who.)
