Sure they cannot be left with unsold 580s that are impossible to sell without a loss. I just don't see why they would have to drop the price in one go, though. That's just not necessary, even considering AMD has new cards to fill the gaps.
The 7950 is really the card we're talking about here, because it's the one most likely to equate 580 performance. The 580 is a bad sell at $500 compared to the 7970. That means that if the 7950 performs just like the 580, AMD aren't going to release the card at $500, that simply wouldn't work. Say they'd release the 7950 at $450 then. There's no reason why Nvidia would drop the price further than $450 then, if it performs the same as the 7950. Even if the 7950 is a bit better than the 580, Nvidia can cut the price by a further 10/20 or 30 bucks no problem. There's no danger of undermining their own cards either, as the 570 is much lower, below $400.
Then what happens is that the 7870 becomes the joker (let's ignore the 7890 for now as I don't believe there's even a rumoured date for it). Depending on how it performs compared to both the 580 and 570, it could undermine them a lot. At rumoured pricing of $300, there's a lot of potential for it to be a much better value. Then again, the cards currently priced around that point also are much better value already.