Shawn McCombs built a DIY RFID Card Lock System for his office. The wiring is a bit quick and dirty but sometimes you just need to get things done. It is based around an ATMega 328 microcontroller and is coded using an Arduino sketch but the controller id hand wired on a breadboard to allow for some customization. Shawn is using the Parallax RFID controller that was designed by Joe Grand (kingpin). Looks like there is a good separation between RFID reader and door control, this is important since with many inexpensive all in one systems the activation relay is built into the reader module. In this case all you would need to do is crack open the reader on the wall and short out the N/O contact to buzz the door strike. If the lock is a fail safe style like a mag lock it is even worse, just knock it off the wall and chop all the wires going to the all in one reader. This will open the N/C contact that is keeping the mag lock energized and unlock the door.