The ATmega8U2 is not listed as a supported device for AVRDUDE, at least according to the list of parts at http://www.nongnu.org/avrdude/user-manual/avrdude_4.html.
I would have tried AT90USB82, as recommended for the Atmel Flip DFU programmer (http://arduino.cc/en/Hacking/DFUProgramming8U2), but that too is not listed.
The discussion at http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=91943 suggests using the -F switch to disable signature checking, and to then use AT90USB162 which apparently is compatible, but again, that part does not seem to be listed as a supported device.
Any suggestions on the best way to proceed?
To put this in context, I am trying to program my Uno's ATmega8U2 using its ICSP interface (which is a different 6 pins on the Uno than for the ATmega328). I am doing this to eliminate the bootloader on the 8U2 which consumes 4K of its flash.