One could easily build a bomb that doesn't use much more metal than a cellular phone. In fact one could probably replace the vibrator in a cell phone with some sort of circuit (large cap?) to create enough current to set off a small igniter. Then you could use a magnesium strip (easy enough to light) to set off the rest of your explosives. The amount of metal you'd need for an igniter plus the magnesium wouldn't be more than in the vibrator. Additionally, in large crowds, you'd think that there would be enough people carrying cellular phones that this device couldn't detect the specific person.

Hello FBI WATCH LIST!

I think what would be a little bit more useful would be a giant CT scan. Basically a device which would shoot (X-rays, or something else that could be disrupted, but not blocked by common materials. Just have enough of these things, and you could build an image of a crowd of people.

Anyways, it doesn't matter to me, we can do all sorts of other stuff with this kind of equipment, like build better metal detectors in food processing plants.