Also, let us not forget that the Alien vs. Predator concept has already been explored, very briefly, on film.
In a scene of Predator 2 near the end of the film, Danny Glover's character enters a Predator ship and sees the "trophy room" with a wall of bleached skulls from the Predators' kills. One of these was from an Alien, implying that these two races have met before.
Another point of interest gleaned from the Dark Horse comic series is that the Predators are responsible for spreading the Aliens from planet to planet, seeding the eggs to create more Aliens for them to hunt. The Predators use the hunt as a ritual undertaken by their adolescents in order to enter into adulthood and to become true hunters.
I absolutely LOVE the idea. I've been waiting for this movie for fifteen years. I sure hope it's a good one.
Mark