It's not officially announced, but looks like the MPAA has handed down an R rating of Ridley Scott's Prometheus. Images of pre-sale ticket stubs, which you can see in the gallery, reveal the big ugly R. That will limit the size of the film's audience, though some may be happy to see the film remain in its original form and resist watering-down by the MPAA. Flixist called it in March.
There have been other R-rated summer blockbusters that made money, like The Matrix Reloaded, Terminator 3: Rise of the Machines, and Bad Boys 2. Hopefully Prometheus has the same mojo, since it is, after all, a return to the Scott vision of the Alien universe. It's probably not even that bad; I bet the MPAA just saw a boob and got scared.