I've attended the Hackfest conference in Quebec city last weekend. There was a presentation about physical pen testing techniques.
The speakers were doing a demo of the Proxmark 3. They showed how "easy" to clone a RFID tag (demo with the hotel access card) in just a few seconds.
You can find an interesting article here:
Getting in with the Proxmark 3 and ProxBrute - SpiderLabs Anterior
Here are some web sites where you can order a Proxmark 3:
http://proxmark3.com
http://www.xfpga.com/