The three prototypes came in today, and I spent some time this evening starting the assembly process. Not all parts are in yet, but they are on their way. So far I have the capacitors and resistors I need, plus several of the Maxim DS-1100Z+175 chips. Soldering these little buggers on the board isn't easy!
Another bit of news... I'm now running the SIO on the Atari XEGS at 57.6Kbps and tweaked the FTDI chip in the adapter to minimize its transmit delay so we can squeeze every bit of performance out of the thing. It's holding up quite well, except for a few glitches that I hope to nail as I solidify the boot ROM and get a stable prototype in place.