Its simple economics. Perhaps its just ONE of the reasons its not being released, but such factors play a major part. Everyone knows that processors would be MUCH faster if intel and AMD weren't milking the market for what its got. The reason speeds come out slowly isn't so much a quality control issue but its because they'll make a lot more money when they release new clock speeds slowly, and let the hardcore users scramble all over it. AMD pissed intel off because not only did their chips have higher yields, so they could ramp up the MHZ higher than the PIII could, but they also had the balls to release those speeds at a much faster pace. Even because of this, Intel wouldn't just relase the most powerful chip they could, they still had a loyal fan base so they'll continue to do so as long as they can stay reletively close to the Athlon.
The same pertains to Nvidia. Since they are the reigning king right now with no apparent competition, they can take their time and milk the market for what its got. This isn't a business about releasing the fastest chips, although most people would like to think so. This is a business for making money. Nvidia is doing just that, and despite the fact that I want an NV20 ASAP, I can't blame them for such a wise decision. There is no need to trip over themselves at the breakneck speed they are going when competition is clearly left in the dust. They're laying back, relaxing, and reaping the benefits of hard work and a great chipset. If intel were in their situation, they probably wouldn't have come close to the mhz range they're operating in right now.
Thank god for competition, though, I only hope 3dfx has the guns to put the pressure on once again and lower those astronomical prices.