More information on the iPhone 5s's fingerprint scanner
The folks over at CITE World took a more detailed look at the technology behind Apple's new sensor:
With the new sensors you don't have to move your finger, just press it against the reader. And like the sensor in the iPhone 5S, the sensors that will be in laptops and keyboards and other phones can detect the ridge and valley pattern of your fingerprint not from the layer of dead skin on the outside of your finger (which a fake finger can easily replicate), but from the living layer of skin under the surface of your finger, using an RF signal. That only works on a live finger; not one that's been severed from your body.
This will protect you from thieves trying to chop off your finger when they mug you for your phone (assuming they're tech-literate thieves, of course), as well as from people with fake fingers using the fingerprint they lifted from your phone screen.
I don't entirely understand how the RF signal differentiates between living and dead skin (maybe it has to do with the water content of the cells in living skin?), but this is some seriously good news! And it means people can't simply lift your fingerprints either.
Cellphone designers and folks designing implanted pacemakers first researched this to avoid excessive RF generation, interference, and damage to living tissue.