Not only will Sonic & Knuckles be coming to XBLA in early September, it will feature lock-on emulation.
Sonic & Knuckles will be coming to Xbox Live Arcade on September 9, 2009, but the real news is that the game will support "lock-on" technology emulation. "Lock-on" is a feature of the bygone cartridge era that unlocked special features in the first three Genesis Sonic titles by literally jamming their cartridges into the top of a Sonic & Knuckles cartridge.
According to the Sega Blog, owning one of the following titles and using "lock-on" in Sonic & Knuckles will do the following:
Sonic the Hedgehog: Unlocks the game "Blue Sphere", the bonus/Chaos Emerald level from Sonic the Hedgehog 3.
Sonic the Hedgehog 2: Lets you play as Knuckles, complete with the ability to get to previously inaccessible areas of the game, and new power-ups added for playing as Knuckles.
Sonic the Hedgehog 3: Play as Knuckles in Sonic 3, and also changes the flow of the game and connects it directly to Sonic & Knuckles, so that you can play the two seamlessly!
Another Genesis title to keep an eye out for on XBLA is the original ToeJam and Earl, which recently won a poll to be Sega's next game on the service. If only we could jam it into the top of something to get better high tops and bling.