SEGA has announced that Sonic & Knuckles are hitting Xbox Live on September 9 complete with the cart stacking bonuses of the original title.
However, you have to have the original games on XBLA first for it to work.
D'toid has provided the full details:
"With Sonic the Hedgehog, you’ll unlock the “Blue Sphere” bonus Chaos level from Sonic the Hedgehog 3. If you own Sonic the Hedgehog 2, then you’ll be able to play as Knuckles in the game. Playing as Knuckles will let you reach areas that Sonic can't reach and will include new power-ups too. And with Sonic the Hedgehog 3, you’ll be able to play the game as Knuckles and the design of the game is changed so the experience between Sonic 3 and Sonic & Knuckles feels seamless."
Sounds like something Sonic fans have been waiting for.
More through the link.