Much of this code was written by a genius named Wolfgang Wieser, who deserves the Nobel Prize in hobbyist electronics. I've tweaked his code in places.
The firmware is at http://will.ware.googlepages.com/fx2_firmware.tgz and the Eagle design files are at http://will.ware.googlepages.com/n1ibt_board.tgz.
I work in Linux, and the stuff in these tarballs is written to work on a Linux box. To build the firmware you'll need SDCC on your machine. I've built the firmware successfully on Mandriva 2006 and Fedora Core 5. I have had trouble getting it to work on Ubuntu and hope to get that fixed because I'm migrating my home machines to Ubuntu.