Sonic and Knuckles will be available on September 9th for download on XBLA; But not without one of its most important features, the ability to use it with other Sonic games you have purchased on XBLA. Owning Sonic the Hedgehog unlocks Blue sphere. Sonic the Hedgehog 2 lets you play the games as knuckles and still includes the extra power ups and before inaccessible areas. Last but not least, having Sonic the Hedgehog 3 will allow you to play as Knuckles as well.
Depending on who you are, this could be a cool addition or just Sega trying to tempt you to buy the other games. The magic of cartridge stacking isn’t actually there but it is kind of neat that Sega tried to embrace it.