Thanks to a California Supreme Court ruling, if you're arrested in the state, police can search the contents of your cell phone at will:
The court cited a number of previous cases wherein defendants were arrested with all manner of incriminating objects—heroin tablets hidden in a cigarette case, paint chips hidden in clothing, marijuana in the trunk of a car—which did not require a warrant to obtain. The court said that the phone was "immediately associated" with Diaz's person, and therefore the warrantless search was valid.
The decision was not unanimous, though.
"The potential intrusion on informational privacy involved in a police search of a person's mobile phone, smartphone or handheld computer is unique among searches of an arrestee's person and effects," Justices Kathryn Mickle Werdegar and Carlos Moreno wrote in dissent.
They went on to argue that the court majority's opinion would allow police "carte blanche, with no showing of exigency, to rummage at leisure through the wealth of personal and business information that can be carried on a mobile phone or handheld computer merely because the device was taken from an arrestee's person. The majority thus sanctions a highly intrusive and unjustified type of search, one meeting neither the warrant requirement nor the reasonableness requirement of the Fourth Amendment to the United States Constitution."
This gives police quite an incentive to arrest suspects and mine whatever can be gleaned from their smart phone. Burners won't be going away any time soon.