From here: http://gizmodo.com/cops-can-make-you-fingerprint-unlock-your-phone-and-th-1653984192
Double-plus ungood.
stay safe.
That sucks, but it's not as bad as it sounds.
Cops can't search your (locked) phone without a warrant. (4A) They can't make you unlock it for them in any fashion without a warrant. (4A supported by SCOTUS). IF they get a warrant, then they get to take the phone and try all kinds of stuff, including using your fingerprints. They can't make you spit out a passcode, because that's "testemony" and 5A protects you from incriminating yourself, whereas a fingerprint is not "testimony".
Any random LEO on a traffic stop fishing expidition can't force me to swipe my phone. The correct answer would be "give me my phone back and get a warrant". Then (in my case) turn off the phone.
The whole phone is encrypted, and it requires a long and annoying password (correcthorsebatterystaple) at startup. I have to balance secure against thieves and pranksters, secure against .gov, and ease of daily use.