There is a wide variety of levels of difficulty for this operation, depending on the model. On my old Lombard, its really easy. On the TiBook, piece of cake. On the 15" Aluminum, its a bit more involved, but quite doable. On the 17" and 12" Alums, it gets more tricky, still doable for the more savvy. On the iBook, I think its too difficult for the average person to do. Easy to mess things up.
I agree with the advice about not voiding your warantee - don't do it. You could always get an external firewire drive if you desperately need more storage.
If you decide to purchase and install a drive, I suggest the Hitachi Travelstar 5K80 series for a great combination of price, performance, quiet operation, and reliability. Stay away from toshiba drives! You can get drives on the cheap here...
http://www.zipzoomfly.com