Hi,
My board model is a UNO R3. I'm using linux (Fedora 16 or Ubuntu 11)
When I plug the usb cable, the green LED is lit, the orange LED flashes. But nothing is visible with lsusb. No message in dmesg. Nothing is listed with dfu-util -l. I tried with 3 different PCs (Fedora and Ubuntu) and with four different cables, no difference. =(
Do you have an idea?
Thank you in advance !
ok, I found a solution 
The firmware was corrupt. To replace it, it is not obvious because the command dfu-programmer is not updated.
If it helps someone:
Download the latest source version (tar.gz):
Download the patch for the atmega16u2 and apply it as explained here:
Download the correct firmware:
https://github.com/arduino/Arduino/tree/master/hardware/arduino/firmwares/arduino-usbserial
Reset the atmega16u2: to do this, briefly bridge the reset pin : the pins are located near the USB connector. Connect them briefly with a piece of wire.
The orange led stays on.
Erase the atmega16u2:
./dfu-programmer atmega16u2 erase
echo $?
The returned value should be 0.
then, flash the chip :
./dfu-programmer atmega16u2 flash Arduino-usbserial-atmega16u2-Uno-Rev3.hex
Validating ...
4034 bytes used (32.83%)
Finally:
./dfu-programmer atmega16u2 reset
The orange LED will turn off.
Disconnect and reconnect the USB
lsusb
Bus 005 Device 008: ID 2341:0043
howdy,
i followed all the mentioned instructions (having the same problem with the sketch not uploading) and all went fine until the flashing.
dfu-programmer erased the firmware (by the way, if using the instructions to apply the patch and compile dfu-prog there is no need for the "./")
but flashing gave me this:
:~/arduino-1.0$ dfu-programmer atmega16u2 flash Arduino-usbserial-atmega16u2-Uno-Rev3.hex
Error opening the file.
Something went wrong with creating the memory image.
any ideas?
it seems it might be something wrong with the patch, but i really can't be shure of that.