Maybe. At this point, it depends where you are (what state) when law enforcement searches your phone whether your personal information in your phone - such as text messages, emails and location data - can be searched by police without a warrant. Suppressing evidence of a crime because law enforcement did not follow the necessary procedures is often a key component of a criminal defense strategy.
A Rhode Island man is currently in jail while the state supreme court decides whether a search of his girlfriend's cell phone that revealed a text message incriminating him in the murder of her 6-year-old son. The girlfriend had called 911 to report that her son was unconscious.
An ambulance took the boy to the hospital with his mother following close behind. Before leaving her apartment however, she'd allowed a police officer who had also responded to the 911 call to come into her home and remain there after she left. The boy did not survive his injuries; the medical examiner blamed his death on a perforated intestine caused by blunt force trauma.
While in the apartment, the police officer read a text on the mother's phone that she had sent to her boyfriend, "Wat if I got 2 take him 2 da hospital wat do I say and dos marks on his neck omg." The phone was in the kitchen of the apartment and was beeping, according to the officer, which is why he picked it up.
The boyfriend was arrested and his cell phone was seized at least in part due to the message on the mother's phone. Although police obtained search warrants for the cell phone records of the boyfriend, the initial evidence that made him a suspect was discovered by a search conducted without a valid warrant.
The Rhode Island Court of Appeals did not extend any exceptions to warrantless searches to allow the evidence against the boyfriend to be used against him at trial. The state is appealing to the supreme court.
Source: The New York Times, "Courts Divided Over Searches of Cellphones," November 25, 2012