Did you sacrifice yourself at the end of the game? No problem. Part of Broken Steel's features is that it lets you finish the game, and keep playing.
Other of Broken Steel's feature are new weapons, new armor, the level cap raised to 30, new perks to compensate the 10 new levels, and 6 new achievements worth 150 points.
Broken Steel is set to release on May 5th, two weeks from today, April 21st.
It will be available for download for 800MSP, and the main story is estimated to be the same length as the Pitt and Operation: Anchorage, however the raised level cap will be sure to add a lot more gameplay.
Article informing the release date and information: http://kotaku.com/5219643/fallout-3s-delivers-puppies-on-may-5
For more information, visit: http://fallout.wikia.com/wiki/Broken_Steel
The achievements can be found on the ^^^^^ website, or can be viewed here: http://www.xbox360achievements.org/game/fallout-3/achievements/
Can't wait.