Audi has come up with a really neat concept car that autonomously drives itself on intermediate road courses. The car has 5 antennas, 1 of which is for radio remote control, 2 for ground based GPS and 2 for satellite GPS. The ground GPS helps the car stay within mere centimeters of its path.
Currently the car uses a pre-programmed path, and it needs the ground based GPS system. But they are working on a way so that the path isn't needed and it can make its own intelligent choices.
There are 2 computers driving this thing, both core 2 duo machines. One is handling sensor data and the other is controlling the car.