If you carry a cell phone, your location is revealed in real-time to your cellphone service. It happens without your noticing it, because the tracking is done transparently as your phone is handed off from cell-tower to cell-tower. You may not realize this, but your cell phone is regularly chatting to the cellphone service giving the service data.
It's a privacy consideration that this is happening, because the information they use can and will be used against you in a court of law.
This article gives a good overview of how it's done:
How Do Cell Phones Reveal Your Location?
(By Brendan I. Koerner; SLATE.COM;
Posted Monday, May 12, 2003, at 2:00 PM PT)
Maybe we should keep our phones turned off when we're not planning to use them ??