stimmer:
when the firmware runs it makes the HWB pin an output (DTR) and it may be high, and connecting it to GND may cause a short.
Thanks for pointing that out. It sounds like the non-soldering section of the tutorial at http://arduino.cc/en/Hacking/DFUProgramming8U2 could do with an update.
With the extra 10k resistor in place HWB is held low whenever it is configured as an input. Does this mean the 8U2 chip goes into DFU mode on powerup? And how do you get it out of DFU mode?