UPDATE: Here is a link to an Arduino Mini shield based on these instructions http://wiki.smallroom.net/doku.php?id=terd:projects:rfidspoofer .
Step 1: Parts
*Some enamel coated solid core copper wire (I used the green spool from the 3 spool set Radio Shack carries).
*A NPN transistor, I used a 2N3904
*A 10 K Ohm Resistor
*A 10 nF capacitor (0.01 uF). I'm using a Metalized polyester film cap I got from Radio Shack, others should work though
*A toilet paper roll to wind the wire on
I tested my circuit using a Parallax RFID serial reader connected to a second Arduino