From Law and Disorder:
The contents of your cell phone can reveal a lot more about you than the naked eye can: who your friends are, what you've been saying and when, which websites you've visited, and more. There has long been debate over user privacy when it comes to various data found on a cell phone, but according to the California Supreme Court, police don't need a warrant to start digging through your phone's contents.
The ruling comes as a result of the conviction of one Gregory Diaz, who was arrested for trying to sell ecstasy to a police informant in 2007 and had his phone confiscated when he arrived at the police station. The police eventually went through Diaz's text message folder and found one that read "6 4 80." Such a message means nothing to most of us, but it was apparently enough to be used as evidence against Diaz (for those curious, it means six pills will cost $80).