Smith v. Maryland and the Fourth Amendment
During a criminal investigation, the victim of a robbery told police he had been getting phone calls from the suspected robber. Without getting a warrant, police installed a surveillance device called a "pen register" at the central telephone office. The pen register recorded the phone numbers called by the suspect from his home phone. When the pen register revealed that the suspect had placed several calls to the victim's phone, the suspect was arrested and eventually convicted of robbery.
The convicted robber appealed, claiming that the warrantless collection of his call records violated his Fourth Amendment rights protecting against unreasonable search and seizure.
Both the state court and U.S. Supreme Court, in Smith v. Maryland, disagreed, ruling that pen registers do not constitute a search in the traditional sense and may be used without a warrant. Since the only information collected by the pen register -- the phone number called -- is automatically shared with the phone company, with the knowledge of the caller, the Court held it unreasonable to expect the information would remain private.
If as they claim, the NSA has recorded only phone numbers, and not conversations, their massive surveillance program represents the world's biggest pen register and would thus enjoy the legal status established by the Supreme Court in Smith v. Maryland.