I think some of the specs that are given in this thread are a bit bogus. Here are the specs for an IBM 120GB 7200RPM DESKSTAR.
Min Operating Temperature : 5 °C
Max Operating Temperature : 55 °C
Humidity Range Operating : 8 - 90%
Shock Tolerance : 55 g @ 2ms half-sine pulse (operating) / 350 g @ 2ms half-sine pulse (non-operating)
Vibration Tolerance : 0.67 g @ RMS (random) (operating) / 1.04 g @ RMS (random) (non-operating)
Sound Emission : 31 dB
So, 55g OPERATING 350g NON OPERATING for shock.
So, data shows orientation of the hard drive may not be a significant factor as it can take 60Gs of shock without failure presumably in any orientation. Getting a hard drive to experience 60Gs of shock when mounted to a 2500 pound car is a feat. The accident that caused Princess Diana's death was 70Gs. So, I eat my words, happy? However, it is only because of the weight of the car that the force was 70Gs. A hard drive which weighs say, one pound, dropped from one foot would experience far more force than this. Thus anchoring the hard drive to the car firmly is imperative.
Vibration tolerance is only 0.67g OPERATING and 1.04g NON OPERATING.
This shows that vibration IS a significant factor. Cases where people are mounting their hard drives rigidly to their car without failure may not be experiencing much vibration. However, this is definitely something the hard drives are prone to failure from.
So like I said before, mount your hard drives in a mount that is rigid enough to take on the mass of the car, but not so rigid that it transfers vibration. This gives the hard drive the greatest possible chance of survival in any condition.