A state court in Virginia recently issued a ruling with significant implications for the privacy of mobile devices. In Virginia Circuit Court, Judge Steven Frucci concluded that police could require a defendant to provide his fingerprint, enabling the authorities to access his smartphone. Judge Frucci also noted, however, that police could not require disclosure of an access code for the device without a warrant.
Judge Frucci ruled that the police could require access to the defendant’s fingerprint. He noted that fingerprints are similar to DNA and handwriting samples in this context. Fingerprints are pieces of tangible evidence, and the law permits authorities to collect such evidence as part of investigations.
In contrast, Judge Frucci noted that pass codes are forms of knowledge. The Fifth Amendment protects defendants from compulsory disclosure of knowledge which might incriminate them. The Judge in this case thus distinguished between fingerprint access controls and traditional pass codes
Police Permitted to Compel Fingerprint Access for Smartphones | Reuters
Ignoring the obvious "it wouldn't work in real life" solution of turning your phone off, forcing a passcode entry on reboot before Touch ID can be used, it's pretty obvious that this ruling is a big bucket of cold water on fingerprints as a security measure against government access.