I got them done at Hackvana (hackvana.com, or #hackvana on irc.freenode.net); similar prices to Seeed, and excellent customer service.
I'm not sure about the maximum speed. The flipflop, for instance, has a delay of 14ns; if we take that as average, each slice has to go through 5 parts (input mux, LUT, flipflop, async select mux, output enable) for a total delay of 70ns, so in theory one slice could do about 14MHz. Since it's a ripple counter, I guess we should divide that by the number of slices, so 4MHz seems like a reasonable upper bound.
It's a really outstanding project. Thanks for sharing it with everyone.
Regarding configuration memory, could you have the arduino read out from a serial flash? They're pretty easy to bit bang or control over SPI, and you can get a 64Mb pretty easily.
I'm guessing you're talking about the SRAM-based alternative I discussed at the end? Yes, that would be an option, as would an SD card. 64Mb would be enough for plenty of 256 kilobit slices.
I do prefer the idea of using EEPROM if I was going down that route, though, so it'd be more like a CPLD than an FPGA. I just need a good way to load the only bit of remaining discrete state - the output enables - on startup. My best idea thus far is to dedicate half the EEPROM to configuration data, store the latch states at address 0, and use two RC networks to create rising edges first on a register latch pin then on the highest address pin to latch in the config before enabling the EEPROM in 'operating mode'.
I'm not sure about the maximum speed. The flipflop, for instance, has a delay of 14ns; if we take that as average, each slice has to go through 5 parts (input mux, LUT, flipflop, async select mux, output enable) for a total delay of 70ns, so in theory one slice could do about 14MHz. Since it's a ripple counter, I guess we should divide that by the number of slices, so 4MHz seems like a reasonable upper bound.