It seems somewhere someone pushed the wrong button, as a listing for an XBLA version of Sonic Adventure 2 showed up on xbox.com today—and then was quickly pulled.
The listed was gone by the time I got to it, but Kokonoe over on NeoGAF managed to grab this screen capture:
The information listing showed the game as having an October 3rd release date, along with offline multiplayer, content downloads, and leaderboards.
Also curious is the last screenshot that was included on the Xbox Marketplace page—which seems to show a PlayStation controller button prompt (as seen below). So, it might not be crazy to also assume that Sonic Adventure 2 will also be coming to PSN.