Two more cents from me.
One concerns panjandrum's post and the Eiger jump he mentioned. These silly jumps we got in GT games (like the Seattle ones) will have to end if damage is implemented. And if it's only because of damage that this "arcade car jumping" in GT ends, then I really want damage implemented.
The second concerns j8mie's post, about the realism of damage. Considering the GT legacy, I guess what concerns more PD about damage is the graphical challenge to make it look realistic. In fact, the most realistic in the PS3. And that is a real challenge, considering the bar was put very high by Grid's excellent graphical damage. IF PD wants to do better, they have to get near photo-realistic perfection.
For those that never have seen it, check this pre-release video, it just shows how the competition is tough, even in the graphical department:
(side note: I don't like Grid, and I don't play Grid, but that happens because of physics, not because of graphics, damage graphics included).
However, j8mie raises two interesting questions about the realism of damage:
a) it will have an effect that has to be realistically rendered, be it in handling, engine output, gearchanging trouble and even in sound;
b) If it is indeed realistic, the slightest mistake can have dire consequences.
About a) I think several games, even from last generation, did a very decent job. One that comes to mid is TRD 3, where the cars would be nearly undriveable, the gearbox would become almost not functional, the engine power would decrease and even the car sound would change. SO I don't see a great problem there.
About b) I guess that, as it happens with the handling option in GT5P, damage will be implemented in a way that you can make it more or less true to real life. At least that's what I hope.