Suggested change to firmware upgrade instructions!
I've successfully downgraded my firmware from 0.67b to 0.62b using the instructions here:http://www.artlebedev.com/everything/optimus/firmware/
It took several tries and I got somewhat confused by the fact that Step 5 only applies the first time you connect the keyboard
in this manner:
In other words, once you successfully install the driver you will no longer get the new device dialog from Windows even if you follow this procedure to upgrade (or downgrade) the firmware again in the future.
I kept thinking that Windows wasn't seeing the keyboard at all, when it was. Launching MassDFUApplication showed the keyboard ready in program mode despite the fact that Windows gave no other indication that the keyboard had been plugged in.