If we're getting weather and day/night transitions it is a must. Obviously you can't simply retexture the entire track every so often during a race (like would happen now with the switches between afternoon/evening etc) so bump-mapping and direction lightsourcing will have to be added if we want proper realism.
Even as a smaller thing, if we want proper projected headlights, these will add another light source to the textures, meaning that soft shadows generated by the sun will be overwritten by the closer, faster moving source created by the headlights.
That was a Dx8 feature, wasn't it? So presumably if your gfx card is Dx8 campatible it shouldn't alter your frame rate that much. In fact, if we did get the switch to hardware-controlled Dx features, frame rates might even increase due to the strain being moved from CPU to GPU.
I'm sure it's all planned out, more or less. It's just a case of waiting to see.