Aliens vs. Predator: Requiem preview at IGN.

"Alien and Predator: they're both mighty, hulking beasts locked in a viscious struggle to outperform one another, and despite everyone's best efforts, pinning down which will be the eventual victor is proving impossible. A bit like the next-gen console war then.

This match-up between the two ultra-violent extra-terrestrials goes back a little way further though. Their paths first crossed in a 1989 comic but most became aware of their rampaging conflict after a tantalising hint in the second Predator movie, which featured a Xenomorph skull on the unlikely piece of furniture that is the Predator trophy cabinet, glimpsed just before the closing credits roll."

Full Source: IGN