they can not be reused because they are programed to the phones they came out of, placed in different phones the phone will say sim card not accepted. there is nothing you can do other then throwing them away or leaving them inside the phone they are assigned to.
http://www.geekzone.co.nz/forums.asp?forumid=39&topicid=79361 is about Sim Cards being reused. . I am curious if Sim cards CAN be reused as I have several old ones from mine and my hubby's old phones. AT MINIMUM they can help to backup your CONTACT list (just used one last night to help ease the process of backing up contacts to switch contacts between cellphones. That was pretty helpful. I don't see why a SIM card can't be reused but still checking. Hope this helps :)