In a unanimous decision today, the US Supreme Court ruled that police need a search warrant to examine the contents of your cellphone. Chief Justice Roberts, who wrote the opinion, said that vast amount of data that we keep on our phones must be protected from routine search.
Roberts wrote, "We cannot deny that our decision today will have an impact on the ability of law enforcement to combat crime....Privacy comes at a cost." He went on to say that cellphones "are now such a pervasive and insistent part of daily life that the proverbial visitor from Mars might conclude they were an important feature of human anatomy....Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple— get a warrant."
Police can still determine "the physical aspects of a phone to ensure that it will not be used as a weapon," but once that determination is made, "data on the phone can endanger no one," and the arrestee will be unable to "delete incriminating data."
The decision can be read here.