Bruce Cahan forwarded me this article written by a colleague of his. It discusses a recent Massachusetts's case in which the government was seeking a court order to require companies to release historical cell site information. One of the primary issues was whether a warrant was required to obtain historical information. (Many, although certainly not all courts, have found that a warrant is required in order to track a person's movements real-time using cell phone data.) The Massachusetts's judge found that a warrant was not required in this case, and that all the government needed to show was that the historical information of a person's movements was relevant to an ongoing investigation.
You will have a good understanding of how complicated of an issue this is after reading the article and the case. I particularly liked the quote from the article about the inability of courts and legislatures to deal with the impact of technology.