I think SFB has a good point, but so does Custard. If you can't monitor your own speed then are you really capable of being safe on the road?
Plus cruise control has been linked to no end of accidents when people forget to switch it off or get complacent about it and lose concentration when driving. Don't forget as well with some stretches of road having so many different speed limits on it you'd be spending all your time setting and resetting the cruise control. It would probably be easier to just use your right foot...
My Big Brother forecast for the future is that we WILL have speed limiters in our cars and they will be controlled by road side transmitters. So when you enter a 30mph zone your car will automatically be limited to 30mph, and same again for 40mph/ 50mph/ 70mph etc.
Brave New World indeed.