Looks like NVIDIA are putting a stop to overclocked G80 cards from partners, according to this report at the Inquirer. They've said their reasoning for this is because they dont want another GeForce 7900 GT overclocking fiasco which resulted in a high return rate for dead / dying cards.
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 Geforce 7950 GX2 and they all worked at the same reference 500MHz clocks as this was what Nvidia guaranteed for these cards.