Inception

To best way to find out whether you're dreaming or not is to ask : "How did I get here?" Because there is no continuity in dreams. People just appear in a certain place or time without questioning why or how. And they only realise it when they wake up after and try to explain them.

So...

Did Cobb dream the entire thing to convince himself of realising his wish to be back with his kids, and to overcome his guilt for planting that idea in Mal's mind?
It'll be like multi-levelled dreams, with the entire thing being played out in his mind alone.

But then again, maybe by ignoring the spinning totem, instead of waiting to see if it stopped or not, he chose his reality - whether or not it is in reality, reality.

Intriguing.