Well, that comparison seems fair, the best thing that nVidia can do is compete in price, considering that with current Catalyst 11.4 shows an average improvement of 7% in lots of games with the HD 6900 series, it ain't hard to prove that the GTX 590 will be indeed slower by a small margin due to being TDP limited. Somehow I suspect that the price launch initially will be the same as the HD 6990 and then it will drop in price first closely followed by HD 6990 price drop.
The 590 will obviously be slower than 6990 at 3 monitor gaming 5760x1200, 5760x1080 etc. But will likely be faster at resolutions like 1920x1200, 1920x1080 etc and the 6990 and 590 will probably be a draw at 2560x1600.
I don't think they will want to price cut the 580 just to release this card, so it will probably cost at least $850 to keep the price of the 580 stable. $150 seems a reasonable savings to eliminate 1 PCB and the better overclocking headroom you'd get out of a single PCB 580.
It makes it a funny card. More power than you'll need and superior performance at resolutions you don't really need the power for.
I don't know that 6990 and 590 will compete too much, one will be the card you want to buy if you are gaming at insane resolutions, 6990, the other will be the one you buy if you want to have the fastest card around at lower resolutions. The former being cheaper, the latter likely costing about $150 more.