A little while back, Apple TV Hacks posted on a great idea: one of their readers, Brandon Holland, is using an Apple TV as a carputer. He chiefly uses it as an in-car entertainment center, but it also does double duty as a GPS system using the Road Trip plugin which he has now released over at AwkwardTV. It can read GPS data from "any NMEA enabled GPS via a serial connection such as a USB to Serial adapter or a Builtin Serial Port. Map Quest maps are obtained via expedia.com and both USA and Europe maps are availiable and working!" Unfortunately, it doesn't yet store the maps locally so it presently depends a network connection, but Brandon says he's working on it, and soon expects to have it "so that map packs or trips can be cached for a trip."
Obviously you'll have to hack your Apple TV to make this work, but given its small size and power requirements, the Apple TV seems like a great foundation for a carputer.