ToastyJaguar on 21 June 2008
I've just found some info that Forza 3 will infact be on 1 disc.
The fact is, no one knows if it will be on 2 discs but this source seems fairly reliable so yeh...
http://theangrypixel.com/blog/2008/06/22/rumor-forza-3-is-coming-sans-disc-swapping/