Ars Technica's take on the launch.

At any rate, using a runtime component to dynamically divide up a workload among a highly parallel architecture that runs at a lower clockspeed (the R600 approach) sounds a lot harder to me than writing directly to a narrower but faster architecture (the G80 approach). I think that this is why the HD 2000 series hasn't dethroned the G80 at the very top of the graphics heap, but it hasn't even tried. The highest-end card announced today is the $399 ATI Radeon 2900 XT, a part that falls well below NVIDIA's top-of-the-line card in both price and performance. The 2900 XT isn't reaching its peak potential with its current software stack, so throwing more GDDR3 and more clockspeed at it for a boutique $1,000 card wouldn't buy it enough performance to stand up to the high-end G80.

At least, that's my current theory.