Forza 3 has finally been confirmed!!!! It looks amazing!
E92 M3 and E82 1 series coupe confirmed already along with the other cars in the videos and screen shots. It comes out this October.
400+ cars and 115 tracks (Not sure if those are individual tracks or lots of configurations like in Forza2)
Check out the site for videos and more info.
http://forzamotorsport.net/en-us/default.htm