The on-board voltage regulator: http://www.onsemi.com/pub/Collateral/MC33269-D.PDF (it seems from online diagrams there are two in parallel, one surface mount, one to-220) claims 20v maximum input voltage. The arduino site says only 6-12v should be used. Would it be alright to connect Vin to a car's 12v line, which varies from 14.5v when the car is on, to 12.1v when the car is off, and down to 8-9v when the car is starting?
It can handle over 20v but its not advised. 14v should be fine.
I'm using one in car and have found that I need at 12V voltage regulator or the arduino will get flipped bits on the chip at random times (more like, when running the car hard). In car voltages apparently vary a lot.