Is that a gun in your pocket or are you just happy to see me?

According to this story by CNN, a machine is being given a trial run at Heathrow Airport which can x-ray a person and see under their clothing. The purpose? To find solid, and potentially dangerous, objects a person might have on them. The response of passengers? Almost unanimously positive. People like to feel safe. The response of civil liberties groups? Voyeurs are looking at your dangly parts!

Ahem. Personally, I'm all about safety and security, though the thought of some random airport security guy looking at my dangly parts does fill me with a certain sense of creepiness. I guess my only question is, what is it that these things pick up that the metal detectors missed? I mean, are those things even good for anything anymore?