Humm, there seem to be low on Sparten III also!
Well, I may have a reason... I talked with someone, at my job, who's in FPGA design (designing with FPGA). We rescently had problems too getting Spartan III, they are entering like 5 per months...
The S3 use 90nm technology, and apparently, they are having very low yield at Xilinx, or more precisely, the company that make their chips (very few chips they make are good). The process used to make those FPGA will be corrected, but when it does, there's already a lot of customers in the list, waiting for Spartan III.
So, that's why they are hard to get. But when everything get corrected, I'm sure Spartan III will drop a lot in price, and let Spartan II in the dust
Think it that way, with 90nm techlonoly (0.09um), you can make chips much smaller, which in turn imply that the manufacturer can put more chips per dye.