[no subject]

    http://en.wikipedia.org/wiki/Telecommunications_data_retention#United_States

The Wikipedia article goes on to comment on the forensic value of data retention. I think it is fair to say that the use of telephone numbers in TV shows like CSI ("gee, he called X a lot, maybe we should too") is the comic book version of the use but not far from the mark. A law enforcement official once described it to me as "mapping criminal networks"; if Alice and Bob are known criminals that talk with each other, and both also talk regularly with Carol, Carol may simply be a mutual friend, but she might also be something else. Further, if Alice and Bob are known criminals in one organization, Dick and Jane are known criminals in another, and a change in communication patterns is observed - Alice and Bob don't talk with Dick or Jane for a long period, and then they start talking - it may signal a shift that law enforcement is interested in.