I have been playing Forza 3 since its release and Ive flipped the car twice only. The cars dont flip as easy as you think.
And in the short time I played it I managed to flip it. it was at the Spanish Montserrat circuit (Is that what it's called?) The same one from the pre-release demo. Gently over a rumble strip and stright onto two wheels, like the car had no weight or downforce at all.
They might not flip very often, but I feel the physics engine exaggerates the forces that cause the car to flip. Maybe PD will implement it better, but then again maybe they won't.
The cars in GT appear to have more weight (But that is because they're stuck to the ground up to now... but the suspension and weight changes all seem to look realistic, but in Forza it's all over the place at times. You can't tell when you're inside the car though). The rollover effects trailer we saw recently shows cars flipping in collisions. The SuperGT crash, where one car hits the back of another and flies into the air, appears to be evidence that cars will lift off the ground should air get underneath them. But we have yet to see if it has been implemented properly. It could be way too glitchy and could happen way too often. It's all good saying Forza has the physics wrong, but nobody can say for sure at this point that GT has these type of physics any better. I don't think the E3 demo had rollover and damage, and that is the latest build we have seen in-game.
EDIT:
We see in many occasions, the Forza rollover is realistic.
Take this one for example:
Obviously, if the car gets onto the grass or gravel, if it digs in slightly it will flip very easily.
I think as for on the road, it's related to how much steering lock you apply. But in some cases, it happens sporadically, with no clear reason or force on the car that would cause it. Like a little magical elf popped up from the rumble strip and pushed your car upwards.