Bethesda's forums are in an uproar about not being credited for the Achievements they earned in the newly released Fallout 3 DLC, Broken Steel.
If you experience the same problem, take heart! Bethesda tells us, "The Xbox Live servers don't seem to be fully synched yet with the additional Achievements. Microsoft is looking into it right now so that the Achievements folks have gotten will show up properly on Live (the game awards them, they just aren't showing up right on Live for some reason). People won't lose any Achievements."
Patience, friends. Sounds like this will be resolved soon.