The title says it all, really. NVIDIA is planning on overclocking the G80 as far as they can get it, which alone wouldn't give you too much a performance increase. However, NVIDIA also has a figurative ace-in-the-hole, a driver tweak. What exactly this driver tweak does is unknown. Whatever it is, though, will give ATI some serious competition when they release their gigantic R600.
http://www.techpowerup.com/?25258