The Genesis seems an odd system to single out for a lack of GameCube controller support, since the system's three buttons would map relatively easily to the GameCube's B, A and X buttons. Personally, we don't relish the idea of spending $40 for two classic controllers just to get some multiplayer Toejam and Earl action going, but we will if we must.
We've contacted Nintendo for confirmation on this matter and will update as soon as they get back to us.
[Update 1: Since launch it has become apparent that Genesis games do work with the Gamecube controller and occasionally with the Wii remote as well. The Nintendo game info. pages have been updated with this information.]
[Thanks to the commenters on this post for pointing us to this story]