Check out this article that describes how the state of Victoria in Australia will be trying out GPS speed monitoring devices that are installed in cars. The GPS technology identifies speed limits on all roads and operates on three levels:
Drivers get an audible warning they are over the limit at level one. At level two, the device cuts power to the engine to prevent the driver from speeding, but the system can be adjusted or overridden. At level three, the system cannot be switched off or adjusted and all speeding is cut.
Not sure Americans would ever go for this! And it seems to me that there are sometimes legitimate reasons to go above the posted speed limit (maybe passing a car?), and all of a sudden having the throttle shut down could be dangerous.