The development team said in the past that the reason damage was not included so far was because they were perfectionists and they would want to model every single possibility so it is as close to reality as possible.
You only have to see the closeness to reality of the real game in order to appreciate the huge effort it will take to implement this feature...
If it took 1 month to model a single car then it would take twice to do the same car with damage, since the damage possibilities are endless...
PSN ID: T_Gears
End of 2009 ltd sales:
Wii = 67-68m
X360 = 38-39m
PS3 = 34-35m
Prediction: The PS3 will surpass the 360 on weekly sales after it drops to $299 on all regular weeks (no big releases).