The readme.txt in arduin-usbserial suggests that the make-process is very simple.
However since it depends on the external LUFA, it would have been great to have a hint which LUFA version to use.
On the LUFA page it says arduinos would work with more recent LUFA versions as well, so I was rather looking (for hours) on the LUFA side for a solution of the problem.
Finally I found the hint in the firmwares directory. Having it as well in the subdirectories would do no harm.
Christoph

Hi,
well THAT's really an easy way! However isn't "touch" and "hold" to be exchanged?

Christoph

Hi Anil,
See http://arduino.cc/forum/index.php/topic,56283.0.html

Christoph

[Preparation]

Hi
I just updated the 8U2 serial firmware of my arduinos according to http://arduino.cc/en/Hacking/DFUProgramming8U2.

If you don't like soldering directly on your arduino board (or better: below your board) you can follow the folowing steps.

Material needed:
 

• One piece of cardboard larger than the arduino.
• A piece of aluminium foil. I was very successful with the candy wrapper of some Lubecker Marzipan.
• One or more arduinos - of course.
• A breadboard connector cable.
• Tools: a pair of scissors.
  

Preparation

• Cut a hole in the cardboard so that you can view the connectors near the map of Italy. For details see http://arduino.cc/en/Hacking/DFUProgramming8U2.
• Put the cardboard on the back of the arduino with the hole in place and cut the cardboard to exactly the size of the arduino. Do not move the cardboard's hole position!
• Stuff the aluminium foil in the hole and smooth it at the other side.
• Turn it over and put the arduino on top. Align cardboard and arduino.
• Connect the cable to Gnd at the arduino board.
• While holding cardboard and arduino steady with some pressure together connect arduino to USB. If you need both hands to establish the connection, you can do so, but take care not to move cardboard or arduino - or at least don't move them too far, 5mm is still OK.
• Touch with the cable the reset pin as indicated on the picture.
• Disconnect the cable and continue updating the firmware according to http://arduino.cc/en/Hacking/DFUProgramming8U2.

Good luck
  Christoph

Yes you can!
See http://arduino.cc/en/Hacking/DFUProgramming8U2

I will prepare in a minute a tutorial explaining how to update without soldering.
Christoph