I just fried my Duemillanove today. I connected the ground to an oscilloscope that had the case ground shorted to the AC line.
This is interesting. Were you powering the Duemilanove from the USB port on your PC?
I just did some tests on my notebook PCs. The USB shields and ground pins on the FTDI
cable seem to be isolated from the AC Mains. I don't see see how that could blow the
mains breaker.