mapAmobile offers a tracking service that will disclose the location of a mobile phone. Here’s some information from their site:
Once you have registered for the service, you can log onto your account using your user name and password. From ‘My Account’ ‘Account Home’ click ‘Where is this mobile?’ from the list of mobiles you have chosen to locate.
After a short while, you will receive a reply online giving the location of the mobile phone: e.g. ‘Gemma is in the vicinity of Carnaby Street, London W1’ together with the date, time and accuracy of the location. You will also have the option to see a map showing street names and points of interest.
It sounds like the mobile device needs to be registered before one can track it. So tracking random people around town is probably not an option (thankfully).
London seems to be a hotbed of location based services, last time we were there we tried out Zingo, a service that puts you in touch with the driver of the nearest taxi. I was skeptical at first, but a taxi showed up within 5 minutes of the call.