As such, if you are arrested or detained by a law enforcement officer, you cannot lawfully be compelled to tell the officer anything other than your basic identifying information—even if the officer has not read you the Miranda warning. Exercising your right to remain silent cannot be held against you in a court of law, nor can it be used to establish probable cause for a search warrant.
However, if you voluntarily disclose or enter your mobile phone password in response to police interrogation, any evidence of illegal activity found on (or by way of) your phone is admissible in court, regardless of whether or not you’ve been Mirandized.
What if you’re not a criminal and think you have nothing to hide? Why not simply cooperate with the police and hand over your password so that you can get on with your life?
For one thing, many Americans are criminals and they don’t even know it. Due to the disturbing phenomenon known as "overcriminalization," it’s very easy to break the law nowadays without realizing it. A May 2010 study from the conservative Heritage Foundation and the National Association of Criminal Defense Lawyers found that three out of every five new nonviolent criminal offenses don’t require criminal intent.
via Ars Technica – Why you should always encrypt your smartphone. Important read for anyone who has a smartphone.