Just in: Supreme Court rules that government generally needs a warrant if it wants to track an individual’s location through cell records over an extended period
If this stuff interests you, I'd recommend getting a copy of 'Habeas Data' by Cyrus Farivar
More, from Ars Technica:
"However, the court declined to overturn the controversial "third-party doctrine," the 1970s-era legal precedent that found there was no "reasonable expectation of privacy" in data collected by a third party, like a phone company."
And then you realize it was 5-4 and easily coulda gone the other way