Forza Motorsport uses a really cool AI system called Drivatar - it not only behaves convincingly (so no more brain dead path-none AI ala GT4) but you can train your own Drivatar that will in turn drive like you. I think the following quote brings home just how impressive an achievement this is:

Think about it … you’ve trained your Drivatar
to drive on 5 different tracks in 5 particular cars and now it is able to drive
all the tracks in the
game in all cars – though effectively it has never experienced them before.

Sounds great, can’t wait.