I admit it - I have an almost irrational fascination with carputers, particularly Mac mini-based ones. I've always wanted to build one myself, but have been stopped by: A) The lack of time and money, and B) The fact that a carputer would probably be worth more than my jalopy.
While we've mentioned a few nice examples here on TUAW, one of the biggest problems has always been the relative paucity of GPS mapping applications for Mac OS X. RouteBuddy has been the leading GPS solution on the Mac in the Intel age, but it was lacking some important features. Fortunately, version 1.3 adds two biggies: turn-by-turn routing and voice prompts. RouteBuddy supports a variety of GPS devices and map data from Tele Atlas.
Unfortunately, it's not cheap. RouteBuddy itself is $99.50US, not including maps. Maps for a variety of countries are available, with US-only maps costing $49.50. A demo is available which includes a street level map of Sante Fe for testing.