I was a little distrurbed by this movie. Hear me out, because I found an article on it after I saw it and that confirmed what I thought to be underlying messages...
First of all - great movie. Not necessarily the most original story line, but incredible 3D and CGI. All movies should be made using this 3D technology! Simply amazing.
Regarding the hidden messages... Perhaps I am too superficial, but I saw this as more of a classic "good vs evil" movie, and I didn't see the America in the evil role. I saw it as an evil corporation using mercenaries against a gentle defenseless population who eventually rise up and defeat the aggressors. This premise has been dragged out ad nauseum in Hollywood - Robocop, Terminator, Transformers, any war movie, and even Star Wars. I certainly didn't see the Galactic Empire as a metaphor for the U.S. of A. You can probably concoct/interpret race or ecology arguments from any number of movies - intended or not. Remember the "Jar-Jar" controversy? Some movies evoke emotions or make you think, and some are simply fun and kick ass entertainment. For me, Avatar was just a lot of fun with great special effects - an enjoyable three hours.