I just finished my project to see if my car is home or not. HomeSeer now has a device that tells me if the car is there. To see if the car is home I’m using a cheap Oregon Scientific Temperature sensor. Combined with the RFXCOM plugin Homeseer can see if the temperature sensor is in range or not.
With this information you can switch devices on/off or have you security cam take pictures. For more info please look at the Car Presence project page.