So many different pieces of information goes into the valuation of stocks, its hard to speculate a reason for it right now. Technology, in general, lags the immediate discussions that happen on forums like this. For example, when Intel's Core 2 came out, the prices were still on the decline for about a week after the chatter on the internet started taking notice of the benchmarks.
Perception and frenzy can also contribute. The economic picture looks dim for the country and many are bracing for a recession because of the popping of the housing bubble. Macroeconomically, most believe average households won't have the discretionary money to spend on computers, cars, and plasma tv's through most of 2008. That could be another partial reason for Nvidia's decline.
And then there's Nvidia's balance sheets. There could be high cost on the new batch of GPU's that may not offset the profits from high sales. So even though consumers rave, analysis of NVDA may show that its expected returns to be smaller than, say, ATI that *might* have lower costs in production and operations.