The GPS clocks are spot on - that's how they work! They use an atomic clock in each satellite so that they can transmit very accurate time information. The receiver compares the delays from each sat to work out its position. More info here:
http://www.garmin.com/aboutGPS/
The system is owned and run by the US Department of Defence so they could pull the plug at any time. Which is why Europe wants to spend a vast amount of money on its own Galileo system:
http://www.esa.int/esaNA/galileo.html