Fallout 3's Mothership Zeta is the last DLC adventure for the (anti-)hero from Vault 101. MTV Multiplayer took note of the wording on Bethesda's website for the game, which states that the latest piece of content is the post-nuclear saga's "final DLC."
We're currently following up with Bethesda to see if they have called it quits on the Wasteland, which means that the next time we'll be hearing from the world of Fallout will be during our trip to New Vegas next year. Come on, Bethesda. There's still some uranium to squeeze from the glowing stone.
Update: Bethesda says "never say 'never,' but this is the last DLC we have planned."