I don't know about that but I'd be fairly certain a court would just order you to unlock the phone regardless of whether it's your finger locking it or a password.
http://www.uclalawreview.org/the-fifth-amendment-encryption-...
>The Fifth Amendment to the U.S. Constitution gives people the right to avoid self-incrimination. That includes divulging secret passwords, Judge Steven C. Frucci ruled. But providing fingerprints and other biometric information is considered outside the protection of the Fifth Amendment, the judge said.
[1] http://blogs.wsj.com/digits/2014/10/31/judge-rules-suspect-c...
A fingerprint, when used on an electronic lock of this kind, is not a key. It's attributable to one person only, not trivially duplicated, and not able to be reverse engineered from a locking mechanism. It requires an action by a single person who cannot be forcibly relieved of their possession of their fingerprint.
Additionally, a key is specified during the manufacturing or assembly of a lock, and comes with the lock, since they are "paired" when the lock is made. However, a fingerprint or password are specified by the user at will after they've assumed ownership of the device. They "testified" their identity to their phone with a fingerprint, just like they did with their password.
If compelled to imprint a finger, it is the same sort of personal interaction that a password entails: the credential holder utters/presents their personal information - not a physical object, but a repeated testimony of the same content they previously and uniquely presented to their device. It should be protected as other self-incriminating testimony under the fifth amendment.
Not quite sure it is though. If they already arrested them, they already have the fingerprint don't they? That is different than key and is different than password.
If you can compel a suspect to stand up on a lineup, or produce id, there's no reason why the court shouldn't be able to compel you to produce a finger.
In technical terms, the finger is really a "something you have" second authentication factor. If you think of it on those terms, it's more like looking at someone's Hardware token than compelling a password disclosure.
So is all your knowledge. The technology to extract it doesn't yet exist, but once it does, should it be deployed by the courts without a challenge from the 5th Amendment?