The device will be relatively simple. It needs to have a lot of 12v inputs and outputs, and a lot of ground inputs and outputs. It will need to be controlled somehow, whether by existing keyfob or by a smartphone app via bluetooth. GPS tracking and a persistant data connection are possible, as well as a camera and/or voice-enabled alarm. Using an arduino as the platform makes it easy to build, easy to modify, and easy to learn how cars work. The entire project will be completely open, including source code and circuit diagrams.
This project is still in alpha stage. I am still collecting parts and working out the arduino code.