Projects like this usually inspire the DIY part of me to get motivated to do something similar, unfortunately, I usually fail and ruin my '94 Geo Metro to boot. The Audi A6 has factory-installed GPS, including the screen shown in the picture. With a little help of a Audi-certified technician, this guy managed to get his Powerbook to display on the GPS screen. Fire up Front Row and he has himself an entire media hub within his car. Best of all, he's using his Sony Ericsson K705i as a Bluetooth remote to control the entire system.
Apple Front Row: Integrated into an Audi A6 [Foursprung]