actually greg I would think it would be OK - although it would be worth checking the model of drive used as the amount of shock a drive can withstand varies enormously (even among 2.5" laptop drives which are designed to withstand higher shocks than desktop units)
you're correct that the drive can withstand much higher forces when off - this is because the heads are safely parked and in no danger of touching the platter. but remember that a laptop with any reasonable amount of RAM would cache large pieces (if not all) of the MP3 file in memory so the HD will only be spinning up occaisonally (laptop drives are very aggressive in this respect to save power)
IBM 2.5" laptop - 150G operating, 700G parked
IBM 3.5" desktop - 55G operating, 225G parked
although those sound high, a drop of just a couple of inches represents enormous G figures. I've got no idea how violent the shocks would be to something firmly attached in the boot of a car - presumably the largest forces would be in the vertical direction as the wheels bounced...
if I can sort out the display end, I'll gamble a laptop drive on a trial
oops - forgot to mention that products such as the Creative Jukebox and the Empeg car player both use 2.5" laptop drives
[This message has been edited by ijam (edited 05 March 2001).]