Anti-American movies are much beloved by the critics and might even get Oscar nominations, but they're poison at the box office.
James Cameron has said that his movie is a criticism of American policy in the War on Terror. There is even use of the term "shock and awe" to decribe an attack on the village of the blue skinned Na'vi, who apparantly have no vices.
The parallel is not appropriate, unless the Na'vi are flying spaceships into Earth cities and destroying them. In Avatar, the evil human megacorporation (Americans) are plundering the planet for energy resources. This works for liberals who believe that the war in Iraq was about oil, but the majority of Americans don't believe that.
If I was Cameron, I would have altered the story somewhat to make it more palatable to American audiences. Make the bad guys French, Germans and Russians and have them making supporting a Na'vi dictator who brutalizes his people in return for oil contracts. Then portray the Americans liberating the Na'vi and throwing the French, Germans and Russians out.
While this might not make the critics happy, the American people would like a movie that makes their own soldiers heroes rather than a movie that portrays American soldiers as villianous pawns of evil megacorporations.
I predict this movie will break even, but not reach the success of James Cameron's previous movies.