According to Ripten, the efforts to repair Xbox Live's problems is relocking Achievements for some players.
The explanation Ripten got from the Xbox customer service rep is that Microsoft has "systematically been rolling back and recovering portions of their servers to weed out the bugs." The end result is that the server might be rolled back to a point in time before you unlocked your Achievement, effectively relocking it.
Getting another Achievement in the same game apparently re-unlocks the one you've already earned, which is great assuming you have attainable Achievements left for the game in question. Maybe I'll just stop playing anything on the 360 until this is all sorted out.
Update: Here's the official word from Microsoft on this matter:
Xbox Live Server Problems Cause Achievements to Disappear [Ripten]
