But what will it do to your pacemaker?

Bruce Schneier:

There are several ways to deactivate RFID-Tags.....Basically [the RFID-Zapper] copies the microwave-oven-method, but in a much smaller scale. It generates a strong electromagnetic field with a coil, which should be placed as near to the target-RFID-Tag as possible. The RFID-Tag then will recive a strong shock of energy comparable with an EMP and some part of it will blow, most likely the capacitator, thus deactivating the chip forever.