With the advent of PREDATORS and the Untitled Alien Prequel coming out, I believe that the timelines have been seperated and made into three different alternate timelines. From what I have heard, Fox has seperated Alien, Predator, and AVP as three different continuity timelines. For me, here is how canon goes..
Keep in mind, I haven't read any of the newer novels, and some of the comics I have not read or some I don't know the year for.
-Rakai'Thwei
Predator Cotinuity
Predator
Predators
Alien Cotinuity
Untitled Alien Prequel
Alien
Aliens
Alien 3
Alien Ressurrection
Alien vs Predator Cotinuity
Alien vs Predator (10,000 BC)
Predator
Predator 2
Alien vs Predator (2004 AD)
Aliens vs Predator Requiem
Predator: Concrete Jungle
Alien
Aliens
Alien 3
Alien vs Predator (Atari Jaguar)
Alien vs Predator (PC)
Alien vs Predator 3 (Set 30 years after Alien 3)
Aliens vs Predator 2 (Set 50 years after Alien 3)
Alien vs Predator: Prey
Aliens: Berserker
Aliens vs Predator: Duel
Aliens vs Predator: War
Aliens vs Predator: Hunter's Planet
Alien Ressurrection