They just need your thumb...

Incorrect. Read my grandparent comment.

http://www.citeworld.com/security/22399/iphone-fingerprint-s...

(Though it may work for a very short period of time if you were alive when they cut it off, but not for long)

OK, thanks, I understand. (I hope we'll never read it in the news "thief cuts off finger".)

If we do, it'll be 'uninformed thief cuts off finger but still fails to hack into phone'.