Jennifer Granick has a piece in Wired discussing a decision from the District of Massachusetts which held that the government can access historical cell records (which can be used to determine location information) with a showing of relevance (as opposed to probable cause, which is typically required).  You can access a copy of the decision here. 

This area of law is laden with statutory complexities and definitions - which I won't delve into here.  Check out the article and the case for more detailed discussions.  For my part, I was struck at how much information about us is "out there" (unbeknownst to us) just for the taking:

Cellular telephone networks consist of a grouping of interconnected "cells." Each cell is serviced by a tower whose antennae transmit and receive signals from cellular telephones within a specific area of coverage. As a caller moves (or roams) through these areas, his or her phone is automatically switched to the tower that (at least in theory) provides the best reception. Cellular telephone companies maintain records of this switching information. The information is used, for among other business purposes, to assess roaming charges. However, the close proximity of cell towers in urban (and some suburban) areas has also imbued cell site information with a value to law enforcement: cell site information coupled with a basic knowledge of trigonometry makes it possible to identify with reasonable certainty the location from which a call was made.

The bottom line according to the court is that there is a distinction between historical data and real time data, which can be used for "present" tracking purposes.  (According to the court, the latter category of information would likely require a higher showing under the Fourth Amendment.)

An interesting case, and interesting discussion.