There is such a device from Amphitech in Canada, and it's approved by Transport Canada for installation. It's on a tour in the Northeastern States right now. Pretty clever - uses millimeter wave radar and a swivelling head to look for wires. The display to the pilot is simple and intuitive - a series of small LCDs on the top of the instrument coaming. If it senses a wire that might be of concern, it shows a green light in the appropriate location. Wire of more serious consequence (there is a good probability you will hit it in the next minute), the light turns to amber, and an extremely serious probability of hitting it turns the light red.
Not cheap (about US$100,000, methinks), but if you regularly work in that environment (EMS, Police) it would probably pay for itself in reduced insurance payments pretty quickly.