None off the GTX 580 Newegg has listed are at $499.- MSRP, so everything is going to be just relative.
About the bart supply, Newegg has out of 14
HD 6850 listed 11 in stock,
there are 11
HD 6870 listed at Newegg and 6 are in stock, that's not exactly bad supply for a card which is right now on Newegg HOT seller list, AMD also has to supply many off the cards for the ready boxes.
Just to compare, there are 11
GTX 580 listed at Newegg and only 3 are in stock, considering GTX 580 is not exactly hot seller mainly because less people buy the top end cards and some off them are most likely waiting for the AMD HD 6900 cards just to compare the performance and value.
Anyway my conclusion is that bart as of today has much better supply than the GTX 580.
As for the GTX 6900 we can only hope there is not going to be any paper-launch as somebody in this thread predicted.