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.
I'm not sure where UK law stands on this issue, but it clearly has wide ranging implications.
And while we're talking about implications of widespread use of biometric security, Glen F has some interesting things to say over at Macworld.com.
XKCD as ever captures this beautifully: