This page:
http://www.arduino.cc/en/Guide/Troubleshooting#upload
says:
The Arduino Uno and Mega 2560 use standard drivers (USB CDC) provided by the operating system to communicate with the ATmega8U2 on the board.
This page:
http://www.arduino.cc/en/Guide/Troubleshooting#upload
says:
The Arduino Uno and Mega 2560 use standard drivers (USB CDC) provided by the operating system to communicate with the ATmega8U2 on the board.