Aliens: Colonial Marines is being developed by Gearbox (of Borderlands fame) and published by Sega. Alien vs Predator was developed by Rebellion and published by Sega. The Aliens RPG was being developed by Obsidian and was canned by Sega.
AvP went first and following AvP we will have Aliens: Colonial Marines. There is currently no release date but it is in development still. Recently several GBX employees have been twittering about level design, and Kevin Riepl has been roped in to compose the score. (Riepl did the Gears of War and Unreal Tournament music).
Despite rumours of a cancellation, I'm pleased to say it's not cancelled and will be with us hopefully at the end of this year or early 2011 (speculation).