Have a look at Major Malfunction's (Adam Laurie) work in this area:
http://hackaday.com/2007/03/25/rfidiot-rfid-io-tools/
http://www.youtube.com/watch?v=3vAvesYoHeo
and many others - good fun stuff, not too expensive to get started
edit: Just spotted this post on Proxclone that you might also be interested in