Zelda Xtreme Forums

A Dutch website by the name of SegaOnline has this screen of Sonic & Knuckles on the XBLA that is supposedly coming to XBLA this summer, and supports "locking on" to the other Sonic games if you have them.

Apparently SegaOnline also reported some other leaks, so this has a good chance of being true as well.

Wikipedia lists Sonic & Knuckles for a Summer 2009 release date.