Doom and Double D Dodgeball are no longer available on Xbox Live Arcade. Noticed by GamerBytes, both titles were pulled with no warning. The games can still be played if they were previously purchased.
Double D Dodgeball's mention in this post is for the simple fact that it happened to occur at the same time as Doom's removal. Speculation as to why Doom was pulled revolves around rights issues involving id Software's acquisition by Zenimax and the XBLA title being published by Activision.
We're following up with Zenimax to find out what its plans are for the game.
Update: A representative for Zenimax informs Joystiq that Doom's departure had to do with "expiring rights" and it "should be back up soon from Bethesda."