PRIVACY COMES AT A COST

The Supreme Court has spoken on the 4th Amendment. "Can you hear me now?" Riley v. California. 
The police need a warrant to search a cell phone, duh. 

The 4th amendment is about reasonableness (...unreasonable search and seizures...) and reasonableness generally requires a warrant to seize evidence. 

Absent more precise guidance from the founding era, we generally determine whether to exempt a given type of search from the warrant requirement "by assessing, on the one hand, the degree to which it intrudes upon an individual's privacy and, on the other, the degree to which it is needed for the promotion of legitimate governmental interests." Such a balancing of interests supported the search incident to arrest exception in Robinson, and a mechanical application of Robinson might well support the warrantless searches at issue here.

But while Robinson's categorical rule strikes the appropriate balance in the context of physical objects, neither of its rationales has much force with respect to digital content on cell phones. On the government interest side, Robinson concluded that the two risks identified in Chimel-harm to officers and destruction of evidence-are present in all custodial arrests. There are no comparable risks when the search is of digital data. In addition, Robinson regarded any privacy interests retained by an individual after arrest as significantly diminished by the fact of the arrest itself. Cell phones, however, place vast quantities of personal information literally in the hands of individuals. A search of the information [*9] on a cell phone bears little resemblance to the type of brief physical search considered in Robinson.

We therefore decline to extend Robinson to searches of data on cell phones, and hold instead that officers must generally secure a warrant before conducting such a search.

Potential harm is a big issue for this analysis:

Perhaps the same might have been said of the cigarette pack seized from Robinson's pocket. Once an officer gained control of the pack, it was unlikely that Robinson could have accessed the pack's contents. But unknown physical objects may always pose risks, no matter how slight, during the tense atmosphere of a custodial arrest. The officer in Robinson testified that he could not identify the objects in the cigarette pack but knew they were not cigarettes. See 414 U.S., at 223 , 236, n. 7. Given that, a further search was a reasonable protective measure. No such unknowns exist with respect to digital data. As the First Circuit explained, the officers who searched Wurie's cell phone "knew exactly what they would find therein: data. They also knew that the data could not harm them.

Rumpole says: The Data cannot cause harm? Ha! Tell that to the defendant whose wife see's his text messages....

More nuggets:

The fact that an arrestee has diminished privacy interests does not mean that the Fourth Amendment falls out of the picture entirely. Not every search "is acceptable solely because a person is in custody."

Cell phones differ in both a quantitative and a qualitative sense from other objects that might be kept on an arrestee's person. The term "cell phone" is itself misleading shorthand; many of these devices are in fact minicomputers that also happen to have the capacity to be used as a telephone. They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers.

...

Mobile application software on a cell phone, or "apps," offer a range of tools for managing detailed information about all aspects of a person's life. There are apps for Democratic Party news and Republican Party news; apps for alcohol, drug, and gambling addictions; apps for sharing prayer requests; apps for tracking pregnancy symptoms; apps for planning your budget; apps for every conceivable hobby or pastime; apps for improving your romantic life. There are popular apps for buying or selling just about anything, and the records of such transactions may be accessible on the phone indefinitely. There are over a million apps available in each of the two major app stores; the phrase "there's an app for that" is now part of the popular lexicon. The average smart phone user has installed 33 apps, which together can form a revealing montage of the user's life. 

...

In 1926, Learned Hand observed (in an opinion later quoted in Chimel) that it is "a totally different thing to search a man's pockets and use against him what they contain, from ransacking his house for everything which may incriminate him. If his pockets contain a cell phone, however, that is no longer true. Indeed, a cell phone search would typically expose to the government far more than the most exhaustive search of a house: A phone not only contains in digital form many sensitive records previ-ously found in the home; it also contains a broad array of private information never found in a home in any form-unless the phone is.

...

We cannot deny that our decision today will have an impact on the ability of law enforcement to combat crime. Cell phones have become important tools in facilitating coordination and communication among members of criminal enterprises, and can provide valuable incriminating information about dangerous criminals. Privacy comes at a cost.

Individual rights and freedoms come at a cost too. See You In Court, more motions to suppress to file.