Seeing Doom on the Xbox 360 with online play was a pleasant surprise and it looks like there are going to be more Doom games coming to Xbox Live Arcade in the future. The ESRB has ratings up for Ultimate Doom and Final Doom, which is a strong suggestion that we’re going to see both of these later. Since Ultimate Doom only had one extra episode (Thy Flesh Consumed) it may be an additional content pack for Doom instead of a standalone game. Final Doom is based off of Doom II, but it has an entirely new set of 64 maps. Since Final Doom has significantly more content it is likely to be a separate release when it comes out.
Edit: It looks like the Xbox 360 version of Doom IS Ultimate Doom, since it contains the editional episode. Thanks to everyone who dropped an e-mail to inform us.