Well, I've just been playing the demo of Forza Racing ... gotta say, it's GT4 minus the drudgery, and plus the extras that the XBox has power to add. The AI is very impressive, folling other cars they make mistakes (locking up the brakes, turning in too early etc), sometimes correcting themselves, sometimes not.
The Viper is actually realistic for a change, use more than 1/2 brake in a corner and the back brakes lock ...