Anyone know what the technology is that tracks people's location by their cellphone signal?

i want to put together a site that uses the same technology as the recently discussed trapster.com where people press a key on their phone to mark a speed trap. the server automatically knows where the user is located by reading a signal sent by the phone and marks the spot on a map for other users to note. my question: anyone know what technology is needed to implement the same technique for a different purpose?