This is interesting and more than a little scary. A service called World Tracker allows the user to display the location of a mobile phone, with an accuracy of 50 to 500 meters on a Google Map.
The cell phone user has to approve the person who wants to track them. The first time they are searched for the cell user gets a text message and must reply to turn on tracking. But just try telling your girlfriend, spouse or parent (for any younger readers) that you'd rather not allow them track you. :-)
It's only available in the UK at the moment, but they plan to expand to other countries, assuming they don't get banned. Now you'll never be able to claim you had to work late again, unless your local pub is beside your office. And don't you dare go within 500 meters of Parnell Street once Stringfellows opens :-)