Microsoft has confirmed that FPS Doom is hitting Xbox Live Marketplace (not Arcade, as was rumoured earlier) - it's due out this evening. You may even be able to download it right now.
Cost of the download is 800 Microsoft Points, and for your money - or points - you get the original single-player game, four-player split-screen multiplayer with Deathmatch and co-op modes and four-player co-op and Deathmatch on Xbox Live.
