Security, eh?
How about fitting a car with mobile phone so that when u want to use the car, u send a time shifting token as sms to that mobile to make it believe it is u that is using the car. If someone else is using the car, the mobile shud send an sms to u, call the police and playback a pre-recoreded distress message and hold the call so that the police can track the vehicle.
(wow, I thought all this?)