well THAT's really an easy way! However isn't "touch" and "hold" to be exchanged?
No. The reason is that when the firmware runs it makes the HWB pin an output (DTR) and it may be high, and connecting it to GND may cause a short.
Holding the first wire on reset makes the HWB pin become a high impedance input. Briefly touching the second wire against the capacitor charges the capacitor, making the HWB input low, so when the first wire is removed HWB is still low and the 8u2 goes into DFU mode. And in case the user has got things wrong, briefly touching the second wire is potentially less damaging than holding it somewhere which might be at +5v.
tim7: Originally I tried to use a method like the one you suggest (you don't of course need the second wire, you can just use the reset button on the uno) but I could never get it to work reliably. Actually I found it more reliable at getting the 8u2 out of DFU mode than getting it into DFU mode. I don't know why that was, maybe it was just my board, or Linux. Now I've soldered on the resistor that the Arduino team suggest so I can't try it any more.