The office where i work like so many other offices has switched its security over to RFID based access cards. From time to time I have misplaced my card or forgot it at home and had to have a new card provisioned. These cards get used dozens of times a day for doors, printers, elevators, etc.
While I do loose the card often, I never forget or loose my phone. So why not put the RFID into the phone.
I started by placing the access card into a small container of acetone. (purchased in a 1liter jug in the fiberglass section of a local automotive parts store) This card had my photo on the front of the card however the acetone dissolved the image almost immediately.
The card slowly starts to come apart in layers over the course of a few minutes. I dont think its possible to leave the card in for to long as the Acetone is only eating away the glue layers between the plastic sheets that make up the card. Once completed you will be left with a mound of plastic with a small antenna/chip that is now separate from the card.
I then took a piece of packing tape and set the antenna on top, I then folded over the packing tape to cover both the top and bottom of the antenna/chip. This will stop the RFID from shoring out on the phone.
Now take the back off your phone (if you have an iPhone this is done with the two screws by the doc connector and sliding the back up) and place the 1mm slim RFID inside.
Put the back on again and you can now scan your phone at any doors/printers/elevators etc.
This whole system worked so well for me I put an RFID on my own garage door. I also have plans to put an RFID into my car ignition so setting my cell down on the dash starts the car. Removing the phone and opening the door would shut the car off.