Bethesda has released a new update for Fallout 3.
The update will refresh the achievements list so it has the new achievements for The Pitt and offer "support for multiple downloadable content." Which would be The Pitt. Again.
What it doesn't do is change anything else to do with the game at all, so don't get excited once you're prompted for the update. The Pitt DLC is due out later this month.