NOTE: This is a modified version of arduino-usbserial, and was only tested on the ATmega16U2, and the below readme is obsolete. On my 16U2 it is hit the 16U2 reset with pliers and sudo make dfu :P
For better instructions (and an image) of how to flash your 16U2 / 8U2: http://forum.arduino.cc/index.php?topic=52076.msg373873#msg373873 Also note that if your board is new enough (atleast the ones with 16U2 are) you dont need to touch/ground the HWB (point near cap), just reset. So you might as well see if it appears as DFU device (lsusb before and after) after only the reset.
To setup the project and upload the Arduino usbserial application firmware to an ATMEGA8U2 using the Arduino USB DFU bootloader:
Check that the board enumerates as either "Arduino Uno" or "Arduino Mega 2560". Test by uploading a new Arduino sketch from the Arduino IDE.