I'm living in Hungary. The film rating in Hungary is: 12, 16, 18 (year above)
The four Alien films was rated 18 what is equal R, all the Prometheus trailer was rated 16, what is equal to PG-13.
I know, the trailer is not show too brutal violence (just the melting head, and other small things), and don't contain harsh language, but these things may in the eventual movie.
I think, Ridley Scott's name is enough, that we get an awsome movie, he is a very precise director, he is making his movies quite attention, so i think, the Prometheus will be R or will be PG-13, but it will be a huge success.