NVIDIA told all of its partners that it simply won't allow the overclocking of G80 cards. It doesn't want another GeForce 7900 GT overclocking fiasco that caused a too high return rate for the cards. Remember, NVIDIA didn't allow anyone to overclock the GeForce 7950 GX2 and they all worked at the same reference 500MHz clocks as this was what NVIDIA guaranteed for these cards. Although maybe one exception remained from Leadtek with their Extreme version. But then again, the main issue related to the high return rate of the GeForce 7900 GT was not linked to the G71 core, but had actually more to do with the GDDR3 memory. Better save than having cards being returned. - The Inquirer

NVIDIA bans overclocking for its G80 based cards

We still beleive that there is a chance that some GeForce 8800 GTX and GTS might end up a little bit faster, as some of the vendors depend on that. Maybe you can overclock the water cooled card if anyone makes one. In the end the NVIDIA card are quite popular just because of way vendors can play around with the speed settings. Many of the vendors have besides the default version also a pre-overclocked one, and via that way pleasing more interested buyers. By limiting the choice, there might always be less interest. We will see it once these GeForce 8800 based cards hit the retail market.