Updating the Atmega8U2 on an Uno or Mega2560 using DFU

http://arduino.cc/en/Hacking/DFUProgramming8U2

The Arduino Uno and Mega 2560 have been shipping with the updated (rev. 0001) version of the ATmega8U2 firmware since around May, 2011. To check which version of the firmware is on your board, connect it to your computer and follow the directions for your operating system:

• Windows: locate the board in the Device Manager (it should be under Ports), right click on it and select "Properties"; in the resulting dialog, click on "driver details" in the "driver" tab. In one the properties, you should the VID, PID, and the revision number.
• Mac OS X: run the System Profiler application (e.g. by selecting "About This Mac" from the Apple menu and clicking the "More Info..." button); go to the USB sheet and click on the item for the Arduino board. The 8U2 firmware version is listed as "version" (e.g. 0.01 for rev. 0001).

If your board has rev. 0001 of the 8U2 firmware on it, there's no need to upgrade it.