I admit it: I was among the throng that helped "Tropic Thunder" supplant "The Dark Night" as the highest-grossing film over the weekend. For what it's worth, I found it amusing but overlong and derivative. The premise -- actors are mistaken for the roles they play -- was the same as "Galaxy Quest," and Tom Cruise's producer character was a raunchier version of Rick Moranis' Larry Siegel from SCTV. I, however, had a (ahem!) professional reason for attending: the movie is making waves on op-ed pages as well as at cineplexes because of its use of the word "retard" and Robert Downey's appearance in amazingly convincing blackface as a Russell Crowe-like Aussie actor whose insistence on verisimilitude leads him to undergo pigment augmentation and to affect a "street" accent even when he's not on camera.

As even some African-Americans admit, Downey's "blackface" act -- which itself is an object of ridicule in the film -- is far removed from the offensiveness of minstrelsy and even from the practice of actors playing people of a different race. The "retard" jokes are a harder case, and I don't quite buy the rationalization that Ben Stiller's performance as "Simple Jack" derives all its laughs from its lampooning of Hollywood's naive lionization of the mentally challenged. Yet it's funny in a sophomoric, guilt-inducing way. I confess that I laughed both at "Simple Jack" and the Downey character's advice to Stiller's character never to "go full retard" if he wanted an Oscar. (An aside: The word "retarded," originally a euphemism for slow-witted, has morphed into an insulting epithet, proving that all politically correct terminology has a short half-life.)

The question, raised in a Washington Post op-ed piece by the mother of a girl with Down Syndrome, is whether black humor about retardation somehow decreases the audience's empathy for real-life retarded persons. I don't think so, but I don't have any retarded relatives. If I did, my enjoyment of "Tropic Thunder" probably would have been a guiltier pleasure -- or no pleasure at all. Then again, if I had just buried a family member, I wouldn't find "Weekend at Bernie's" a laugh riot.

The photo of protesters demonstrating at the Los Angeles premiere of "Tropic Thunder" Aug. 11, 2008, is by Robyn Beck/AFP/Getty Images.