This caused me a lot of grief and took me forever to figure out, so I thought I'd pass on the warning. When you're binding signals to pins (Edit Constraints) during synthesis, make sure you name the pins P. The P is important. If you don't have it, the Xilinx software will spit out some errors as it's working. If you're like me, though, and don't watch the output too closely, you'll miss them, and the Xilinx software says everything is okay when it's done. It wasn't until I had the EPROM burnt and was trying to test my circuit that I found that many of my pins didn't have any signal (high-impedence).