Roy,
All the cigarette socket extensions with a USB port on them that I have come across so far have stated on them that the USB outputs 5V as per USB standard, you can get adapters that actually plug into the cigarette socket which also contain a USB port but they are 5V as well. The same goes for the Cigarette socket to mini USB adapters that I've seen, so I'm not sure whether this is really a problem, the USB port circuitry has a voltage dropper in it to reduce the voltage to from 12V to 5V. Have you had any direct experience of these sockets blowing up USB devices?, if so I'm positive we'd all like to know what is at risk and from what blocks.
One other note, beware of the output current, most of the USB ports in the cigarette socket blocks are only 500mA (1/2 amp) which isn't enough to charge an iPhone for example, you need an adapter capable of outputting 1A to charge an iPhone.
Cheers
Whizz