SEGA just announced that 7 classic Genesis games will be porter over to Xbox Live Arcade this summer. The list includes, Gunstar Heroes, Shinobi (arcade version), Sonic 3, Sonic and Knuckles, Altered Beast, Phantasy Star II, and Comix Zone.
Note that all of these other than Gunstar Heroes was released in the Sonic's Ultimate Genesis Collection disc. So, we would highly recommend purchasing that rather than these individually. Unless the newly added Xbox Live Support for these games is enough to make you double dip.
No pricing details or exact dates were revealed for any of these titles.