Fallout 3's first DLC, "Dead Money" will become available on December 21, 2010. The DLC will add a new adventure to the game. You are tagging alongside three other captured wastelanders to recover, 'the legendary treasure of the Sierra Madre Casino.'
According to the press release, "in Dead Money, your life hangs in the balance as you face new terrain, foes, and choices. It is up to you how you play your cards in the quest to survive." You can almost hear that guy who does the movie trailers reading that.