Adults should have the ability to process the jokes employed by "Tropic Thunder" within the context of the film and recognize that the joke doesn't target those with disabilities. It satirically takes aim at actors who exploit roles in which they play disabled characters in order to garner acclaim and win awards. Since children might not be able to make that connection or understand that context, the R-rating serves a strong role.
The trailer looks hilarious, though. Can't wait to see it, but that probably means once it's on video. JM