Attack of the Clones "starring" YodaFOX Germany sent a copy of the Episode II credits to the guys over at Episode-X.com. Browsing through the credits, I noticed something interesting. Under "Starring:", it has the following:
and FRANK OZ as Yoda
This makes it look like Yoda has one of the four largest roles in the film. Then, under "Cast", Yoda is sixth, behind (in order) Obi-Wan Kenobi, Padme, Anakin Skywalker, Count Dooku, and Mace Windu. Yoda will have a prominent role in Episode II, there's no doubt about it. I think that for the other films, Yoda and Frank Oz have been under "Co-Starring".