Here's a way to get the Uno into DFU mode without any soldering. Put two wires into GND pins on either side of the board. Then put the other ends of the wires on the board as follows:
The sequence is, hold the first wire on the top left 8u2 ICSP pad, then touch the second wire against the left side of the capacitor below the RX LED. Then remove both wires. It's slightly different to the sequence in the instructions but it seems to work.
Be careful with the second wire as the capacitor is quite near a 5v track. You can try using a low-value resistor instead of a wire if you are worried about blowing up your board.
To get back to normal mode, hold wire 1 in place as above, press the reset button, then remove wire 1. Or just unplug the USB cable and plug it back in.