Here is a new idea for revision 2. Create a 12 button key pad with the single analog pin method and add connection points to add a RFID reader to it. Ads more useful applications to the shield.
EDIT:
Found that the AnalogButton library in the playground will work with this. The example given has all the resistors lined up in series but it works with this method of connecting the resistors as well. But doesn't allow for button combos. The Library will need to be edited to add this functionality to it.
http://www.arduino.cc/playground/Code/AnalogButtons