That's like hoping that a new Learjet will drive down the price of subway tickets.
By which I don't mean that ATI's cards are any worse than Nvidia's -- just that Fermi is going to be ridiculously overpriced and won't drive down the price of anything. I'm betting that they'll be $600 when they release, and people will still buy them just because they insist on having Nvidia or on having the Latest Big Expensive Gadget (they will trade in their crossfired 5870s to get this).
I'd still wait a couple months, though. The price of 5870s is due to come down on its own, simply because most of the people willing to pay $400 for one have already done so.