Elisabeth Hulette, of the Virginian Pilot (via Steven Frank because, really, why would I be reading the Virginian Pilot?):
Judge Steven C. Frucci ruled this week that giving police a fingerprint is akin to providing a DNA or handwriting sample or an actual key, which the law permits. A pass code, though, requires the defendant to divulge knowledge, which the law protects against, according to Frucci’s written opinion.
You may disagree with this — I know I do — but this argument actually makes sense. A workaround for this, if you’re interested, is to simply shut off your iOS device before the police seize it; it will require the passcode when it wakes.