As much as we'd like to flatter ourselves by claiming to be the first to notice the Mac mini's car stereo-sized form factor would be a perfect fit, it was an idea that probably was obvious to anyone who has installed a car stereo (or in our case, made sandwiches while watching someone install it). And lots of people are ready to stick one in their dash (or elsewhere), since its relatively low power draw and small size would make a great car computer. Classic Restorations have already installed a couple.
But the missing element to turn the Mac mini into a proper car PC is a proper front-end software, something I am excited to see being taken up by the 'iDash' project. They're just getting started, but are aiming for an open-source, modular Applescript front-end that will allow people to pop a Mac mini in their car, hook up a touchscreen, and go. They've got a lot of work to do—integrating external hardware, developing software module standards, setting up Bluetooth cellular voice and data connections, etc.—but it's got promise.
Project: iDash: Summary [SourceForge]