The Supreme Court has ruled on the implied privacy of calling records, but that isn't the issue. The National Security Agency, a part of the Department of Defense, is not chartered to conduct domestic surviellance of any kind. If it has been rechartered to do so, that is an issue that should be debated. It's illegal for the NSA to conduct any programs involving domestic communications.
Why? Because (here comes the simplest language I can find):
MILITARY SURVIELLANCE OF CIVILIAN TELECOMMUNICATIONS IS THE IT EQUIVALENT OF MARTIAL LAW.