Don't worry, completing Broken Steel won't end the game either. The Broken Steel story picks up two weeks later, with you dealing of the consequences of your actions. The original Fallout 3 endgame has been changed, so that three companions will no longer refuse to aid you in your final task.
It also increases the level cap from 20 to 30 and introduces some new high-level perks, armor and weapons. Broken Steel takes place after the original ending of the game, allowing you to continue to explore the world after the main quest line is complete. Not only does Broken Steel add a new main quest line which, at four to five hours, is longer than The Pitt, but it also removes a lot of Fallout 3 fans' major qualms with the game - the ending. So, you've taken part in Operation Anchorage and visited The Pitt and you're still hungry for more Fallout 3 content? Luckily you don't have too much longer to wait for Broken Steel, the third - and largest - piece of downloadable content for the game, which will be made available on Xbox Live and PC on May 5.