From the Blues post about the transmogrifier and the void bank it seems that you will need to have the item you want your piece to look like and the void bank is a way to store it without taking up space. Not a bad idea but it could be annoying for pieces that can't be gotten any more (eg can old Naxx gear still be gotten elsewhere?)

What annoys me about it though is they can expand the db to fit all the new tables and relations in for this but they still can't give us more slots per server.