Kind of depends what you have to pay for them.
At the moment the GTX 580 commands a premium because its 1. New, and 2. Has the title "Fastest GPU". Waiting until this card has competition in the marketplace (again, single GPU/card) in the form of AMD's HD 6970/6950 should result in a lower price -which in turn will translate into lower prices for the GTX 480, and also offer an alternative to the nvidia card if the performance level is similar (which should be the case).
The principle reasons for going with the GTX 580 over the GTX 480 or 470 would be the decreased noise and heat output due to a more efficient GPU and cooler design. If noise and heat aren't a concern - or you plan to use a waterblock- then the one metric that makes sense is whether the 10-20% performance decrease in the GTX 480 (for example) is matched by a 10-20% decrease in cost.
Bear in mind that the GTX 570 (basically a cooler, quieter GTX 480) is due for release fairly soon. This will also impact pricing on the existing (but soon to EOL'ed) GTX 480/470.
My personal advice would be to wait until the GTX 570, HD 6970 and HD 6950 make their appearances. You'll have a wider choice, better pricing, and at least another very fast card to take into consideration.
If your scope is not limited to one card then Crossfired HD 6870's will give better (for the most part) performance at a lower price with comparable power usage.
