Square-Enix is going to be a full third party. Games are planned for Revolution, Xbox-360, and PS3. Joygasm for all!!!
http://cube.ign.com/articles/615/615089p1.html
""Miyamoto is challenging his teams to create something new." His new creation will debut with Revolution. Big franchises will of course be there -- including new versions of Mario and Zelda. Short (pre-rendered) Metroid Prime 3 clip showing Samus standing in front of her redesigned spaceship. "We expect strong third-party support in both hemispheres." For example, Square Enix is already working on Final Fantasy: Crystal Chronicles for Revolution."
I know I read this in other places too, but this is just one link I found easily.