You may remember—or not because it happened so quickly—that Doom was yanked from XBLA almost as quickly as it was put there. Even more strangely, no reason was ever given as to why.
That has been corrected as of today. Doom is back for your nostalgic gameplaying joy for only 400 MS Points.
If you need more to humor you about Doom, then view the following video about a father Doom-ing his son for oversleeping.