We sell a Vehicle Tracker, a very similar product to your idea. Essentially it's a GSM mobile phone, data card, battery, movement sensor and some simple intelligence. The GPS plugs into it to provide current location. At any time you can ring it from a modem equipped PC to find the current location, download the history of where it has been or track it in realtime. You can also set a geo-fence so if the vehicle moves outside (or inside) the fence it will send you a text message. It can also be hooked up to a couple of digital inputs, maybe the alarm or a pressure sensor and again it'll text you to let you know.