I am willing to bet the DVD format cant hold enough hires textures etc... and since XBox360 is not going to natively support an internal HD-DVD drive.. they are SOL.
Notice all the fanboys posting "Well - I am sure it will still be awesoem with reduced cars, tracks, features and number of on track cars" -- they have been bumped frm 12 to 8 cars at once.
I thought the XBox360 was some heavy weight hardware - but rumour has it that 60fps was only possible with 8 cars at once.. after all the 12 cars at once hype...