I was searching Slate and happened upon this interesting article, which discusses how your movements (current and historical) can be tracked using your cell phone.
How Do Cell Phones Reveal Your Location? By Brendan I. Koerner:
"...Location data extrapolated from tower records is frequently used in criminal cases. It was vital, for example, to the prosecution of David Westerfield, who was convicted of murdering 7-year-old Danielle van Dam in San Diego. The killer's cell-phone usage revealed a bizarre travel pattern in the two days following the girl's disappearance, including a suspicious trip to the desert. In cases like this, wireless providers will not release a user's records without a court order, save for rare instances in which a kidnapping has taken place and time is of the essence...."
One thing that the article did not highlight is that as long as your phone is on, it is regularly communicating with the local towers, generally checking into the netwok and checking for messages. This information can be logged and often is. So even if you aren't talking on the phone, it can reveal your location.