I was just on the IMDb boards and noticed that user, Crewsally, had posted a link to a page on the British Board of Film Classification. According to that article a 23 minute Making-Of Alien vs Predator Documentary (for a potential DVD) has gained a 15 certificate in the UK.
This has definitely lifted my spirits as I presumed a PG-13 rating equals a 12-certificate. This means that the movie must at least receive a 15 in the UK but there is still a chance that it will get a PG13 in the US.