Remember that PG-13 was pretty much single-handedly created because of Steven Spielberg and George Lucas. The fact that "Indiana Jones and the Temple of Doom" was so brutally violent in parts (can you imagine seeing a heart being ripped out in a PG film these days?) and still rated PG caused a lot of parents to get upset that their children were seeing "real" violence in a film that was judged "OK" for everyone. Of course, this was back in the day when parents actually gave a flip about what their kids were watching and didn't just use Nintendo as a babysitter, but I digress.
Anyway, Spielberg was in on the creation of the rating (since another film he produced, Joe Dante's "Gremlins" was the other "PG offender") so films that weren't necessarily violent or gory or sexual in nature but that weren't something you'd want little Johnny watching at a matinee could still be released without the stigma of an R rating, which means no kids could get into see it (theoretically).
I've become a rather harsh critic of the MPAA's rating system these last few years (why are films that have any hint of nudity or sexuality given R ratings while films that are excessively violent given as low as a PG? How is a movie like the last Austin Powers a PG-13 film and not an R?) and I've grown to loathe the mentality of movie studios that will have director's cut their films down, even as early in the script stage, in order to avoid an R rating. Case in point is "Alien vs. Predator" (which I will not discuss the quality of said film, which I felt was about one step, quality wise, above a porno, and the porno at least would have been interesting), which had a PG-13 rating. All four "Alien" movies and both "Predator" movies before it had R ratings. Alien vs. Predator should have as well.
Anyway, my point after all that nonsensical rambling is that Lucas has the power to make the film he wants without a studio interfering. If he cuts the film down in tone in order to get a PG, it will contradict his manifesto of doing what he wants outside of the Hollywood system and not letting anyone else influence the stories he wants to tell. Thankfully, for all the hell he catches from AICN-esque sheep for "raping (their) childhood", he has integrity and I think the movie will be what it needs to be, PG, PG-13, R, or NC-17* and as long as it is what he wants to tell, I'll be happy.
*no Padme/Lightsaber jokes, please.