So I purchased the IOIO from sparkfun.com and modified the HelloIOIOService app to listen for a state change on one of the digital pins (+5v=HIGH, 0v=LOW) using a voltage divider (discussed here) with input coming from a harness in the car that gives +12v when ignition is on and 0v when it's off. This alone is a fairly expensive way of turning the screen on and off with ignition (although, it allows for much more complex logic than something like the relay solution.)
I also just purchased an SPI-to-CAN module to make the IOIO earn its keep.
I'm hoping to glean some information about how to interact with it from some existing projects. I think all of the info is there.
Arduino + Controller Area Network (CAN)
Hacking Your Car (loads of info)
Interesting Presentation on CAN (with security considerations)
IOIO SPI Doc