I believe that the 'Aussie Series' incorporated into Forza 3 will be based on Bathurst. It will be a battle of the Fords and Holdens from what I heard.
I doubt there will be any other fiesta models other than what is already mentioned but with 400 cars proposed and downloadable content to follow, we'll never know until it finally comes out...
I personally can't wait, I loved the first 2 forza's....