I was quite impressed with this guy's minimalistic aproach to RFID reader and especially his RFID tag.
http://scanlime.org/2008/08/simplest-rfid-reader/
http://scanlime.org/2008/09/using-an-avr-as-an-rfid-tag/
He used Propeler mcu, but it shouldn't be too difficult to port it to another platfom.
by Duane Benson
by Jake Hertz
by Jake Hertz
by Duane Benson