I do not think we will ever have something that limits a cars speed to say 70mph for the reasons Chris said. Besides most cars already do have a governor on them...albeit a very high one.

What I could see being proposed is a method to shut a car off when involved in a high speed pursuit. Whether this device is installed in the car, or some sort of device that is hand held by the police, I am sure we are going to see this technology emerge if it already has not.