It seems that another of Sonic's retro titles is getting a release on Xbox Live Arcade, and possibly on the PlayStation Network as well. A page from the Xbox Live Marketplace briefly showed Sonic Adventure 2 before being pulled, complete with screenshots and release information.
A NeoGAF user (via CVG) spotted the page and grabbed the screens before it was removed. The game lists October 3 as its release date, and mentions online leaderboards and a remastered HD graphics. One of the screens even shows what appears to be a Circle button prompt from a PlayStation 3, leading to speculation that it will be available on PSN. Sonic Adventure 2 introduced Shadow the Hedgehog, Sonic's gun-toting doppelganger.
A similar Xbox Live Marketplace leak a few months ago revealed that a Monster World Collection was coming to Xbox Live Arcade. Plus, the original Sonic Adventure has been available on XBLA since 2010, so it makes sense that the better-regarded of the games would make the jump too.