You raise a good point, and really, only time will tell. The hard drive in the G4 is rubber mounted (good soft neoprene rubber), but I still think enough vibration would kill it.
One idea is to replace the internal hard disk with a Compact Flash card (+ IDE Adapter). CF cards are up to something like 8GB now (heaps of room for the OS, etc) and you could then mount an external USB hard drive in a more vibration friendly manner to store your music, etc.
S.