He wasn't dreaming at the end, the top wobbled. Considering the slow zoom in on the top, if the filmmaker intended anyone to interpret the ending as dream, the top wouldn't have wobbled. The wobble was just to elicit groans from the audience as it started to fall, and made us start cranking our minds to figure out what happened.
The simple and non-ambiguous ending is that he isn't in a dream because Saito shot him, waking him up. The last scene before Cobb woke up was Saito picking up a handgun. This was Cobb's plan all along. He sent Fischer back to reality after planting the idea (inception) that Fischer would be able to break up his father's empire. This allowed Cobb to live a life with his kids. Well, his creepy motherless kids.
There are many more complicated theories, but I think this is the intended "obvious" ending.