The spec on the Arduino website actually lists 12v as the upper limit of the 'recommended' range. The absolute maximum is 20v, although I get the feeling it wouldn't last long at that voltage.

Given there's no heatsink on the reg I'd be reluctant to use it in my car like that given the normal operating supply voltage with the engine running is approx. 14.5v. If it was running all the time I wonder if that could reduce the lifespan of the device.