Satoru Iwata (Nintendo's president) apparently said (of Nintendo's next console) in an interview with Japan's
Famitsu magazine about a year ago...
Quote
"It will be able to use the games already created in the previous generation. And it is something that will work."
Of course, there have been a lot of rumors flying around, saying that the Revolution will be some sort of crazy absurd system like nobody's ever seen before, and it won't have a controller, and will be powered by brainwaves, and all sorts of other crazy stuff, so we really don't know anything. But I think it's a safe guess that it'll at least be capable of playing GameCube games, and it'll at least have a port to plug in a GameCube controller.