First, let me just say: I haven't seen Alien vs. Predator, and have no intention of seeing it in a theater. I've heard too many bad things about it, but here's the main one:
Alien vs. Predator fills me with dread. What bothers me isn't the concept, it's the director. The Alien movie series had always had a tradition of getting young, talented, not-very-well-known (at the time) people to direct them: Ridley Scott (Blade Runner, Thelma & Louise), James Cameron (Titanic), David Fincher (Se7en, Fight Club), Jean-Pierre Jeunet (City of Lost Children, Amélie). For AvP, they got Paul W. S. Anderson, whose resume includes such works of cinematic art as Mortal Kombat and Resident Evil. Which makes me suspect this is more of a videogame marketing movie than anything else.
The concept itself has an interesting history. In Predator 2, there's a scene where Danny Glover's character ends up inside the Predator's spaceship, and finds a "trophy room" – a wall of skulls. If you look carefully, in the quick pan across the wall of skulls, you can see that the set designers included, as an in-joke, an elongated skull such as one would get out of one of the Alien movie aliens (sort of like how, in Star Wars: The Phantom Menace, Watto's junk dealership has a pod from 2001 in the pile, and there are some E.T. aliens visible in the senate). Just a cute gag in the background for the geeks to spot.
Meanwhile: Dark Horse Comics acquires the comic-book rights to the Alien series. A couple of years after that, they also, coincidentally, acquire the comic-book rights to the Predator series, and they release some comics in both of these worlds, fleshing out a lot of the background of these alien species. Now, since comic-book companies tend to have a lot of geeks on staff, someone there remembers the Alien skull in Predator 2, and they come out with an Alien vs. Predator comic. People who were less geeky, and hence were unaware of the in-joke reference in Predator 2, see this as just a cheesy marketing gimmick, but the comic sells well anyway.
Well enough, in fact, that [whichever company it was] worked with everyone necessary to get the license to produce videogames based on them. People who were even less geeky than the comic fans, and hence were unaware of either the Predator 2 reference or the comic books, are completely confused by this Alien vs. Predator concept that just came out of nowhere, but the games sell well anyway.
Well enough that, now, it's come full circle and there's a movie. People who are even less geeky than the videogamers, and hence were unaware of all of this history, see this as a completely cheesy marketing gimmick, and inevitably compare it to the Freddy vs. Jason movie. A comparison which, in my opinion, is unfair: Freddy vs. Jason really was just a case of "hey, we own both of these horror franchises, let's mush 'em together, that should sell some tickets."
My only issue is that instead of following the Alien tradition and getting a bright young visionary director, they seem to have just hired whatever hack they could get to work quick and cheap, which just reinforces the belief that it's a cheesy marketing gimmick.
Now, just don’t get me started on that Will Smith Fights Robots movie (I refuse to call it by the name they used).