An addition to this:
Due to the stackable shield nature of the Arduino, it would be easy to make this extremely modular, allowing non hardware people to add only the features they want simply by plugging in a new daughterboard, and downloading the appropriate module. I am working on some software that will allow custom firmware to be put on the board at runtime, while allowing a very easy to use UI to configure it. For example, a gui would allow you to essentially give it this logic:
When Input 1 goes high (Ignition on) I want Output 2 to go high for 1 second (Turn on carpc)
And press "upload" and all that is left is plugging in your wires.
Just an example as a possible use:
Press a button on your carpc if you want your car to start at 5:00am in the morning the next morning (I go to work at the same time every morning, this would be useful for me)