In case anyone ever needs to program an Arduino Mega using avrdude but doesn’t want to run around the Internet looking for the command line settings, here you go:
Arduino Mega (ATmega 1280)
avrdude -p m1280 -c arduino -P /path/to/serial -b 57600 -F -U flash:w:/path/to/image.hex
Arduino Mega (ATmega 2560)
avrdude -p m2560 -c stk500v2 -P /path/to/serial -b 115200 -F -U flash:w:/path/to/image.hex