Fermi is engineered to have 3 BILLION transistors... and is a much larger chip than what AMD made. Of course it will be faster... but it'll also use more power, generate more heat, and cost more money. Fermi also seems to have the similar bandwidth bottlenecks as the AMD chips, suggesting that this will choke the card out of performing too much better than AMD, despite an obvious advantage in processing power. It'll be faster, yeah, but I think they're shooting themselves in the foot and will deliver inferior price/performance ratio.
Considering the die size, I would expect Fermi based GT300s to be priced too high for the majority of gamers to give a damn about them... a competitor to the 5850 seems ages away, IMHO... and by the time the green team has one, AMD could have better performing refreshes of the 58xx parts.
With this kind of lead... AMD'd have to basically fall asleep at the wheel to let Nvidia win any meaningful battles with GT300.