It depends on your RAID system. At work we have a few Digital(now Compaq(now HP)) StorageWorks RA7000 RAID arrays, with dual HSZ70 RAID Controllers. Using the command line interface to the HSZ70 controllers, we can add and remove drives from an array. In fact, I just did this in December to our mail service array, to replace a failing hard drive. This was done while the array was online, with no down-time for the mail service.
Of course, a StorageWorks RAID Array system is nowhere near cheap. You have to use StorageWorks controllers, and StorageWorks hot-pluggable drives with this system.