Can not put Arduino UNO R2's atmega 16U2 into DFU mode

Development Tools IDE 1.x
10

hiduino:
You need to verify what UNO board you have again. There are three basic versions of the UNO.

The original UNO uses the 8U2 chip for USB. To put this into DFU mode you need to short two different locations at the same time. This original version board has the 8U2 chip in a horizontal orientation.
See http://arduino.cc/forum/index.php/topic,111.0.html

The UNO R2 also uses the 8U2 chip for USB. But it has added a resistor to pull-down the HWB signal. So all you need to short is the RESET to ground on the 8U2 to put it into DFU mode. This version board has the 8U2 in a diagonal orientation.

Only the UNO R3 uses the 16U2 chip for USB. This also includes the resistor pull-down for the HWB signal on the 16U2. So all you need to short is the RESET to ground on the 16U2 to put it into DFU mode. This version has the reset button next to the USB connector.

Please verify your board version and try the appropiate steps.

As you are correct, AVRDUDE will do a chip id "SIG" verification based on the .conf file. "Expected signature..." and such. The 8u2 and 16u2 will sort either way. It's all good.