If you compare the 350z in Prologue, to the 370z in the GT Academy time trial demo, you will see there is a big difference in the physics model. It allows the rear end of the cars to step out properly (If you play both you'll see what I mean - Although if you didn't play the TT demo then it might be difficult lol). But the Rear wheel cars in GT5P stepped out like they were on a pivot and the front end moved in as the rear moved out, which isn't exactly realistic, the rear end loses grip and it the energy/torque/power (Whatever you want to call it) going through the wheels pushes the back end sideways, when the front end will still grip. This was the case in the Time Trial demo. If this works for all cars it should allow you to get the back end out on some 4WD/AWD cars too (Though you'll have to push it harder than RWD cars).
Forza 3 does it right, but the grip levels are exaggerated so it takes a little more throttle to get the wheels spinning on most cars (And it's far too easy to correct, the wheels regain grip too quickly when the throttle is lifted). I'm not sure about 4WD/AWD cars in that game, i've only driven one of them, the R8 (Which has a 2/3 rear bias, not really a good comparison).
