Digit-Life has posted up an article covering improvements (?) to Pixel/Vertex Shader performance in 5x.xx NVIDIA Drivers versus the previous 4x.xx Drivers.

Check it out. I've mentioned this a lot in a to-be-posted review of a GeForce FX 5600 Ultra & indeed it does seem NVIDIA have managed to improve performance a decent amount, though it's still unlikely they'll get near ATi.