It's called a memory shortage. Same thing that held back the NV40/R420 high end parts upon release, and the same thing that's caused so many other videocards to take forever to show up in number. Samsung can only produce so much ram you know, and with no one else being big in the GDDR memory game it's samsung or nobody. They have the cores, it's the ram that's holding them back. Besides, they don't need that many of them as next quarter is another videocard refresh! Much like how ATi's last orders for the X1800XT are for *THIS MONTH*.
Memoy shortage is also the reason for the price of the 512MB GTX. 512MB of 1.1ns ram is the reason the GTX is so much more expensive than the X1800XT.

Its surprisingly also the reason ATI's X1600 is priced so high. THe X1600 core (its the same for either XT or PRO) is very cheap to make for ATI. There are X1600PROs with DDR1 for ~$130 whereas a card w/ the same core and 256MB of 1.4ns GDDR3 is priced at ~$180 to $210.

Perkam