It looks as though Major Nelson, via his blog, has inadvertently announced the existence of a Sonic CD port for XBLA. Considering the fact that some Sonic fans consider Sonic CD to be the best iteration of the franchise, this is pretty interesting news.
Buried casually in between a list of other, already known releases, Sonic CD is listed as being available to play among games such as Trine 2, Ms. Explosion Man and Trials Evolution.
At the moment we have no idea when the game will be released and at what cost - but at a rough guess we're expecting 800MS points and a release before Christmas.
We're currently chasing with Microsoft for confirmation.