The USB Virtual COM Port driver from ftdichip.com won't install, I suspect that GlobalScale modified the hardware/vendor ID in the firmware of the JTAG console first and then didn't or couldn't obtain Windows certification for modified drivers later.
This whole JTAG daughterboard thing smells to me like an attempt to "proprietize" the plug in order to extort an extra 30% profit margin, because the GuruPlug is a brick without it. You don't dare modify the installed environment without a working serial connection, because you'll be hosed if the plug fails to boot due to a typo or something in a configuration file.
And speaking of boot failure, it appears that they intended to implement a cascading boot sequence [WiFi first, Ethernet second] but botched it. Inserting an ethernet cable causes the plug to reboot, but it doesn't recognize the mode change and just goes into an endless reboot cycle until you pull the cable and let it boot as an AP. (I commented out # /root/init_setup.sh in /etc/rc.local to get it to boot Ethernet successfully, after editing /etc/network/interfaces to add:
iface eth0 inet static
or if I wanted dhcp, I'd have added
iface eth0 inet dhcp
(these instructions per An introduction to Debian networking setup
I then had to restart networking with /etc/init.d/networking restart, but it did (eventually) come up.
I didn't have a Fedora box laying around (and didn't want to build one) so I used a MythTV box (Ubuntu) instead. The system auto-detected the guruplug serial cable, loaded the (standard) Ftdi_sio.ko module and left a link in /dev/ttyUSB0 to access it. I used PuTTY to SSH into the MythTV box, apt-get installed minicom, configured it per the Globalscale "documentation", and it worked. (PuTTY and WinSCP are indipensible for this).
That's all I've figured out so far, except -- oh, yeah -- there's evidently no eSATA support in uBoot, so we'll be building one of those too, it seems.
U-Boot 2009.11-rc1-00602-g28a9c08-dirty, indeed.