On November 29, I argued that the legal authorities should have gotten a warrant if they wanted to access the information on where Timothy Carpenter made calls from. The basis for my argument was that getting the information without Carpenter's consent violated his Fourth Amendment rights.
But the records the police obtained where not Carpenter's records; they were the phone company's records. The law is well established (Smith v. Maryland) that when you share your information with a third party -- like a bank when you write a check, or the phone company when you dial a phone -- you can no longer say it is your private information. Exceptions, of course, exist under specific laws protecting school records and health records, but not phone records.
I think Phil is right that "you can no longer say it is your private information."
Five days after I wrote, my Hoover colleague Richard Epstein, an actual legal scholar (I just occasionally play one on EconLog--in this case, to my regret) laid out the issues nicely. I recommend his whole article, "Reasonable Searches in the Digital Age," Defining Ideas, December 4. This is actually an issue on which Richard managed to change my view substantially. My view is, at a minimum, in flux.