For reference sake:
There is a way to fix it, and it works...
As supplied by the SMC support people.
With all cords to the router unplugged, you need to jumper the two pins sticking out inside the router (next to the USB connector).
Then copy the Firmware directory from the supplied CD to your computer. Restart in Dos-mode with the USB connected between the router and computer, and plug in the router. Then run the 'flash.bat' program from the firmware directory and press enter when it asks for MAC addresses.
Once it has finished, remove the jumper and the router is back to square one.
I didn't have a jumper, but a staple did the job just fine...