(More images of ID-12 in the lab here)
I have had the chance to work with the ID-12 RFID reader for a couple of days. It is very small, very cheap and easy to use. I have tested it connected to a arduino microcontroller, to a computer with a usb-serial adapter and wirelessly to a computer with a bluetooth modem.
You can find a code example for maxmsp + schematics on how to hook up the reader here.