I can't speak for everyone, but for me it's a price/performance issue and a performance/watt issue, and in both cases the GTX 580 falls flat on its face. I got my 6950 for $230 last February. Even when GTX 580's have gone on sale for as low as ~$420, I didn't bite. The reason is my 6950 overclocked is just as fast (or faster) than a GTX 580, and consumes less power, for half the cost. Why would I waste money on a GTX 580? For the chance to overclock it and maybe gain another 20%? That's a waste of money. Fast forward to now, with the 7970 for $50 more I'll be getting a guaranteed 25% boost at stock, and hopefully 60%+ once overclocking kicks in.
Yeah I understand, I can overclock my cards beyond stock 580 SLI performance, so we're on the same page there. Power draw of 6 series went up considerably with voltage and clocks if I remember right though, so the power draw difference may not have been as drastic as you thought.
Currently you can get 580s (and I wouldn't get them due to the price/performance aspect and my current resolution) for $450~ that's before (or even if) Nvidia drops the price to respond (they may not have to, at least not much). Assuming some markup on the $550 MSRP it's going to be closer to a $150 or more price difference.
Which brings me to my biggest problem with the 7970, for me at least. The performance per watt is really nice, even the increase over stock 6970s was pretty decent. However knowing how this stuff works, I'm aware that this is just the first step into improved performance per watt. I know that AMD is price gouging hard because they're doing very poorly as a company. I also know AMD took a hit on their performance this gen because of the GCN redesign, but even so they saw a 40-60% increase at times over the 6970. Which leads me to believe Nvidia should see as good if not better performance increases over the 580. Also it leads me to believe Nvidia is going to show decent improvements in their power vs performance area, the 580 was faster than the 6970 and consumed ~75 more watts at full load, while the 570 was much closer only drawing ~25 more watts. If Nvidia gets the same performance gain (and they should actually get more over the 580 than the 7970 got over the 6970) while retaining something close to what AMD got for power/performance it's going push the 7970 down into the upper $300 price range.
Huge block, sorry. However since Nvidia doesn't want to talk about "when" it could be this Q, or next, or next year for all we know, but we should all be aware the 780 is going to trounce this card.
What I'm trying to say is this card is poorly priced, unless you absolutely have to upgrade, and do it now, and can only do so with this one card it makes very little sense to get it now instead of waiting it out.