While I was on the official website I came across the following in the "Ask the Lucasfilm Jedi Council" and was answered by Rick McCallum and can be found here
Q. Will there be any appearances in Episode III by classic trilogy characters like Chewbacca or Tarkin or Mon Mothma?
A. No, I don't think we'll see any beyond the characters in Episode II, like Artoo and Threepio.
Sorry if this has been talked about before (This section isnt one I normally come too).No Mon Mothma no Tarkin?,Whats going on? Of all the people THEY should be in the film!