Maybe a spoiler and of course some of you may already know this, but I came across it on another forum...
First of all, Yoda travels to the Wookiee's homeworld of Kashyyyk, in Episode III. Anybody expecting an Endor Planet will be in for a surprise. Kashyyyk will be more lagoon like. Yes it will have trees, but it will also have Ocean's, lagoons, swamps, jungles and marsh's. Basically a cross between Endor, Dagobah and Kamino.
The reason for Yoda going to Kashyyyk is not clear, but as Episode III begins, the Wookiees and the Clones are fighting the Sepratist force's. They will get rid of all Battle Droids-Presumably with Yoda's help-But around the middle of the movie, we will see the Clones being turned on the Jedi. As the Clones turn, the Wookiees will protect Yoda. During this period, Yoda may or may not, do his Crazy Yoda impersonation from ESB, to try and fall the Clones that he isn't the great warrior they are looking for. In anycase, eventually Yoda survive's the clone attack, but several Wookiee's are dead. One remains from the group that was defending him-Chewbacca!
Chewie takes Yoda to the capital of Kashyyyk (which I believe is supposed to be amazing-Like a spaceport built on top of a Jungle) Via the scenic route. As they fly through the lagoon/jungle/swamp on vines we will see Yoda on Chewie's back-Very remincant of Luke and Yoda in ESB.
There are a number of reports that along the way, Yoda will come across a temple and find something of major significance, relating to Anakin being the Chosen One and the Sith. This part is VERY sketchy, though, and has always seemed far-fecthed to me. Although, if Yoda is looking for something specific, it could help explain what he's doing on Kashyyyk in the first place.
Yoda will arrive at the capital city, and get a shuttle back to Coruscant. Chewie will watch him go, and that will be the last we see of the Mighty Chewbacca, until he turns up with Han Solo, in A New Hope!
Thats what I've heard, but as always, if I hear anymore, I'll let you know.
again sorry If you've read it before.