I just bought a perfectly new, including fresh out of the box of an Arduino Uno R3, but the screens for devices has it showing as a Atmega 16u2 DFU.
The usb-to-serial firmware isn't loaded.
I do know that when plugging in the other one, an Arduino Duemilanove, that the system "twangs" and the system picks the FTDI one and it works properly.
Since the Uno doesn't use a FTDI chip, this step really doesn't tell you anything other than the USB port works correctly.
I think I've got a device which needs to be returned to the store for replacement.
Personally, I would return it. However, you could try re-flashing the 16u2:http://arduino.cc/en/Hacking/DFUProgramming8U2