AMO's lead writer has posted a detailed article on the ending of Inception and why it wasn't all just a dream:
"People who have seen Inception will know that it is all about dreams. Where things get even more complicated is the idea put forth of dreams within dreams. Thankfully, Nolan and DiCaprio manage to convey these ideas with minimal convolution, instead making it all seem like a natural part of the narrative. Dom Cobb (DiCaprio) only has one desire left in life: to be reunited with his children. It is this driving force that compels Cobb to delve deeper and deeper into the world of dreams. It is also the reason we follow him."