I've created a circuit to use an Arduino to monitor the on board communications used by a MINI (R53 2002-2006 model years, as well as some BMW's) for low priority information (ie. stereo controls, steering wheel controls, CD Changer information, window open/close states, etc) via it's internal I-Bus data bus.
In this hack, I used the Arduino to monitor a hacked/pre-made I-Bus communications module. The Arduino looks for specific I-Bus codes triggered by pressing otherwise unused Multifunction Steering Wheel buttons (unused unless a rather expensive factory Cellphone/Blue Tooth adapter is installed) and upon seeing the appropriate code will trigger/switch some external hardware.
As a result, I can now mute my Radar Detector and control an iPod's Play/Pause, FF, Rev functions from my steering wheel without having to alter any of the stock wiring.
Details are posted on this MINI forum thread if you are interested: http://www.northamericanmotoring.com/forums/showthread.php?t=155161
Information on the I-Bus used on some older BMW's and MINI's (2006 model year and earlier) for internal communcations can be found here: http://autos.groups.yahoo.com/group/HackTheIBus/
Most people on the Hack the I-Bus group have used a full blown Car PC to interface with the I-Bus. In this case, for simple control of some hacked external hardware, the humble but very capable Arduino more than holds its own.
Thanks again for a great hacking resource. Arduinos rule!!!