The flash PROM can now also be programmed using xc3sprog. It has taken a lot of experimenting to get the PROM programming to work. Xilinx has not released details of the programming algorithm for these devices in its datasheets. This means that I cannot be sure that the delays used are correct. Also verification is not performed.
Negatives aside, this program has sucessfully programmed the Flash PROM on the $99 Spartan3 Starter Kit.
Regards Andrew Rogers