Prior to the empeg, I had a JVC Kameleon headunit that could do this. They call it the "Audio Cruise Mode" and the manual describes it as monitoring the alternator current to detect the speed. It also lets you change the sensitivity and set the "idle" speed.

It was pretty handy, even for a fairly quiet car.

The moral of the story is: there's no crazy factory radio voodoo or audio sampling behind this technology. It can be accomplished by monitoring electrical signals.