Our cases have recognized that the Fourth Amendment was the founding generation’s response to the reviled “general warrants” and “writs of assistance” of the colonial era, which allowed British officers to rummage through homes in an unrestrained search for evidence of criminal activity. Opposition to such searches was in fact one of the driving forces behind the Revolution itself. In 1761, the patriot James Otis delivered a speech in Boston denouncing the use of writs of assistance. A young John Adams was there, and he would later write that “[e]very man of a crowded audience appeared to me to go away, as I did, ready to take arms against writs of assistance.” According to Adams, Otis’s speech was “the first scene of the first act of opposition to the arbitrary claims of Great Britain. Then and there the child Independence was born.”
Modern cell phones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans “the privacies of life.” The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought. 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.
Also (and thanks to commenter L Pseudonymous for the pointer),
[T]he Government proposes that law enforcement agencies “develop protocols to address” concerns raised by cloud computing [and the risk that searches of phones incident to arrest will access even information that's not on the phone itself -EV]. Probably a good idea, but the Founders did not fight a revolution to gain the right to government agency protocols. The possibility that a search might extend well beyond papers and effects in the physical proximity of an arrestee is yet another reason that the privacy interests here dwarf those in [earlier physical search-incident-to-arrest cases].