But really, the thing is, is that if someone is willing to mutilate your body to get into your phone, it'd be faster, easier, and less messy for them to just put a gun to your head and have you unlock it on the spot.
I think you're right. For that matter, just make them put their finger on the Home button and unlock it.
Someone else here had a great idea, btw... let the user designate a finger that means you DID NOT want to unlock it; that you were in trouble. If you use it, the phone should open a fake homescreen and invisibly signal the police.