R. Baunach asked if the inertia of a moving car can hurt a spinning hard drive.
The inertia of normal driving shouldn't cause any harm. People use their laptops on trains and buses all of the time--some of the buses in my area even offer WiFi.
I used to own an MP3 player with a hard drive, and I played it frequently--plugged into my car's sound system--while driving. That didn't seem to bother it.
But the sudden deacceleration of an accident or near-accident could cause a passenger to lose control of their laptop. If it slams into the dashboard, you could end up with an ex-laptop.
And the likelihood of your experiencing such sudden deacceleration goes up exponentially if you're using the laptop while driving. If you slam into a tree while surfing the Web at 60mph, your shattered laptop will be the least of your worries.