The world is certainly not lacking devices that allow you to track and control your vehicle from afar. The newest breed of these systems was developed by Inilex, which allows a subscriber/user to do everything from unlock their doors, start their vehicle, sound the alarm, disable the engine or give the location of their ride. The system can be used by logging on to Inilex's website via a PC, calling a toll-free number or using a PDA with access to the internet.
Aside from providing a bit more convenience and protection, the major draw for some users, namely parents, is the ability to not only track the movements of the vehicle, but also be alerted via text message or email if the kiddies stray from set boundaries or operate the vehicle during certain times of the day or night (no more ditching school or sneaking into the love interest's window at 3 AM).
Straightline has several links that provide more information and Inilex has a demo posted on their site.