Okay...this whole race thing is getting...yeah. But really. As the show is ANIME, everybody is asian-based. The changes in eye color are not an indication of race. Neither is the size of the eye, that is simply for characterization (to make Aang and Ty Lee more innocent, childish).
Someone mentioned that the city of Ba Sing Se looked caucasian. Assuming that they meant that the PEOPLE of the city looked caucasian, I would have to disagree. Just take a look at the Dai Li.
http://upload.wikimedia.org/wikipedia/en/5/55/City_of_Walls_and_Secrets-_Dai_Lee.JPG
And Aang is unmistakably asian. Take a look at this picture of him as a baby.
http://upload.wikimedia.org/wikipedia/en/9/94/Aang-baby.jpg
Caucasian? I think not. And look at the rest of the airbenders.
http://upload.wikimedia.org/wikipedia/en/b/b5/AN_Monks_5.png
Also, I think everyone is forgetting that the movie is BASED ON the tv show, not an exact replication. People keep mentioning Jonathan Nyugen and Aang's age difference, but in the movie, Aang could be a 16 year old boy. I am guessing that they will play up the romance with Katara in the movie, after all, it is Hollywood, and casting a goofy 12-year old would be pretty strange if Katara is about 15. Remember, the show is ANIMATED. Real life is very different, and the rules change.
The Avatar tv show is a great show, but in translating it into a movie, a lot of its elements will most likely be lost, or adjusted. There will probably be less silly moments, and the movie will probably be more serious. I am expecting an action-packed movie, not a comedy.