Prior to the release of the Ben Stiller comedy Tropic Thunder, speculation was rampant that a huge controversy would be generated by Robert Downey, Jr.'s portrayal of a white actor playing a black soldier (in the movie within the movie).
Nope. That controversy hasn't erupted. Instead, advocates of the mentally disabled are calling for a boycott over a subplot concerning an action hero, Tugg Speedman (Stiller) who is distraught that his turn as a mentally disabled man didn't garnish any awards. A fellow actor (Downey) counsels that Oscars only come when going all the way in portraying a person afflicted with retardation. As evidence, he points out that Sean Penn went "full retard" in I Am Sam, and lost out. Dustin Hoffman, in Rain Man, and Tom Hanks, in Forrest Gump, won big because they went halfway.
Critics generally have agreed with Stiller's contention that he is making fun of self-important actors, not making fun of the disabled.
That scene sounds harsh, and I'd hate for those viewers who are unable to grasp Stiller's satiric intent to be encouraged in their use of a hurtful word. I haven't yet seen the movie, so I can't really weigh in.
But here's a good L.A. Times piece, by John Horn, on the controversy. He includes snippets of reviews. And here's more on the subject, from MTV.