In a unanimous decision on June 25, 2014, the United States Supreme Court in Riley v. California ruled that the police generally may not, without a warrant, search digital information on a cell phone seized from an individual who has been arrested. This case overturns the California Supreme Court case of People v. Diaz (2011) 51 Cal.4th 84, which held that the Fourth Amendment permits a warrantless search of a cell phone data incident to an arrest, so long as the cell phone was immediately associated with the arrestee's person.
The Fourth Amendment is alive and well as it applies to cell phones and the Supremes were right on point. Most cell phones contain a wealth of information about the lives of ordinary Americans. If the police seek to search a person's cell phone, then they must obtain a search warrant.