You realize that you don't even come close to doubling power consumption when making a dual GPU card. In the past it's always been close to a 50% increase in power consumption. Compare the GTX275 and the GTX295 as well as the 5870 and the 5970. Both right around a 50% increase in maximum power draw. So two 470's would be pretty close to 300W. But this is also just all speculation, won't know for sure until the cards actually arrive and people can test them.
http://www.techpowerup.com/reviews/ASUS/EAH5970/29.html
You can't compair the GTX275 single to the dual GTX295. The GTX295 is essentially two 275 chips on on PCB. However Nvidia clocked each core down to the GTX260 clocks to control heat and power consumption. Trust me I know. I own one. If I overclock back to 275 levels or above the card starts to artifact after some intense gaming.
Not only WILL the power consumption be higher then the ATI equivelent. The performance will drop off as well. There is no way the are putting two 480's on one PCB. They would have a hard time with cooling with two 470's. and even then they would have to clock them down. Then would still be higher than a 5970 for power consumption.
So with the current architecture a dual GPU card from Nvidia could not compete with the 5970 right now. It's just a fact. It would be to expensive, too hot and draw way too much and most likley render a lower performance then two 480's in SLI.
Your right it's all speculation and we will not know until one is made. But I just can't logically see one until we see a die shrink.