This reminds me of a security conference I was at a couple years back. One of the presenters was talking about using retinal scanners to identify tank gunners only allowing certain personal to fire the weapon. A young lady stood up and asked "what would stop me from cutting of my enemies head and using his eyeball to fire the weapon?".

The presenter responded, "you miss, have a great future in military weapon design!" His point, you have to think a little out of the box. Most optical scanner actually require blood running through the eye in order to perform the identification. A severed head wouldn't get you anywhere.

While it would be possible to lift a finger print I believe it would difficult to use that finger print in scanners that require you to have a certain temperature (wouldn't work so well up here in Canada at the corner gas station) or measure your pulse for instance.

Some design options also combine "what you have" (a card) and "who you are" (biometric). If either one doesn't match up, you're rejected.