I'd rather not have to basically pay a $100 premium for a 5850 or 5870 because Nvidia doesn't have anything comparable at the same price.
If they did, I'm sure most would agree that a 5850 would probably be priced about $199 and the 5870 about $299 (the initial prices for the 4850 and 4870 respectively). And that doesn't even include the additional premiums both cards command due to short supplies relative to demand. If Nvidia had comparable cards, there wouldn't be shortages on ATI cards and we'd be seeing rebates rather than price hikes from retailers.
It's a lose-lose situation, even if you are an ATI "fan."