The G71 GPU will likely sample next week, but until now none of NVIDIA's partners has seen the chip whereas all of ATI's partners have had the Radeon X1900 XTX for some time now.
Nvidia is very tight lipped about its new 90 nanometre flagship chip and it really needs some very high frequencies to beat R580. It's not a question whether Nvidia made it work at very high speeds at even higher than 700 MHz but it needs to get good yields out of it as well. It's all about the yields and Nvidia just cannot afford to have one more ghost product.