Quartavius Davis was convicted of robbing, in 2010, seven stores in and around Miami and sentenced to roughly 162 years in prison. His prosecutors based their case, in large part, on cellphone records that placed Davis near the scene.

The evidence included records of the cell towers to which their phones were connected when they placed and received calls, according to court documents.

These data were obtained solely on the basis of a claim “that the records were relevant and material to an ongoing investigation.”

The 11^th Circuit demurred. Writing for a unanimous court, Judge David Sentelle wrote in part

[I]t cannot be denied that the Fourth Amendment protection against unreasonable searches and seizures shields the people from the warrantless interception of electronic data or sound waves carrying communications. The next step of analysis, then, is to inquire whether that protection covers not only content, but also the transmission itself when it reveals information about the personal source of the transmission, specifically his location.

And [emphasis added]

One’s cell phone, unlike an automobile, can accompany its owner anywhere. Thus, the exposure of the cell site location information can convert what would otherwise be a private event into a public one. When one’s whereabouts are not public, then one may have a reasonable expectation of privacy in those whereabouts.
…
There is a reasonable privacy interest in being near the home of a lover, or a dispensary of medication, or a place of worship, or a house of ill repute. [W]e do not see…Davis’s location outside his expectation of privacy.

And ultimately,

Cell site location information is within the subscriber’s reasonable expectation of privacy. The obtaining of that data without a warrant is a Fourth Amendment violation.

Yewbetcha.

A 4^th Amendment warrant requires a showing of probable cause, a rather stricter standard than just the government claiming an interest. However, the matter isn’t closed with this ruling; the 5^th and 6^th Circuits have ruled that warrants are not needed in such cases. This points to an eventual Supreme Court case.

The 11^th Circuit’s opinion can be read here.