People can spin this any way they like. It seems logical to me that while the GTX 580 is a great card, I do not believe they are out of stock solely because they are so popular. They are selling well, but nVidia cannot keep up with the demand at this point.
1) Christmas Gift - $500 for a Christmas present is a lot of money, I doubt that this is going to be a present for another person. Very rarely have I seen someone give another a $500 gift just for Christmas. I could buy that it was for themselves, possibly or a married couple.
2) Value - The $350 570 GTX provides so much more value* over the the 580 GTX. For a 50% increase in price you get a 15-20% performance increase. Is 20% going to make a game playable versus not? I doubt it. But it is possible. The stock levels of the GTX 570 are very healthy and since it is hard to believe that a 580 would outsell a 570, I have to believe that the 580 is in short supply.
I don't think you can spin it any other way. It was not a paper launch, they had stock. But they cannot keep up with the current demand, that is surely smaller than the demand for the GTX 570 and the GTX 570 has plenty of stock.