San Francisco - Location, location, location - that's been the time-honoured mantra for property buyers for centuries. But now it's getting personal.

In case you haven't noticed, hundreds of millions of people around the planet now carry personal tracking devices around with them every day. Their cell-phones broadcast their location all the time. Tech firms and marketers see it as a huge opportunity, but privacy advocates are squirming at the implications.

The worries grew this week.

Web giant Yahoo on Tuesday unveiled an application called Fire Eagle that allows users to easily share their location via their mobile phone with friends, internet programs, their home automation system or anything else that's connected to the world's vast digital net.

Yahoo has made the program freely available to any developer who wishes to use it. A tour-book company could sell online tours that self-narrate over the phone as users move from landmark to landmark on a visit to London.

"For years, we have been talking about location-based services as the next frontier of the internet," says internet development consultant Tim McCullen. "Fire Eagle is a huge step in making that happen."

Yahoo didn't invent location based services. GPS navigation devices already offer drivers numerous options. But the feeling is that with devices like the iPhone spreading the mobile internet, the sector is about to take off.

Already 50 programs are incorporating the service into their applications.

A start-up called Loopt allows you automatically broadcast your location to selected recipients on a real-time basis. Blogging platform SixApart allows users to automatically geo-tag their locations, and the Doppler social network allows frequent travelers to share their locations.

Of course, there are more obvious uses for LBS programs such as finding the nearest business or service, such as an automated-teller machine or restaurant, navigation aids, and the tracking of people, vehicles or traffic.

But the major beneficiaries could be advertisers, who are drooling at the prospect of sending promotions to mobile users based on their locations - alerting them to special discounts at nearby stores, for example.

The use of location-based services is moving beyond the internet. California regulators recently passed Pay As You Drive insurance legislation that would allow insurance companies to place tracking devices in cars and calculate rates according to actual mileage driven.

Not surprisingly, privacy advocates are deeply concerned about the implications of the new technology.

"Where I drive, when I get there and whether I stop on the way is not the business of my insurance company or any other corporation who wants to place eyes in my car," says Carmen Balber of the group Consumer Watchdog.

Yahoo counters privacy concerns by noting that Fire Eagle differentiates itself from other services by the ease with which it allows users to control what information is released about them and to whom.

But that's of little comfort to privacy advocates who note that most people do not delve down into software programs to customize features.

"For individuals who do not want their location to be known, these services could be harmful," said Beth Givens, director of the Privacy Rights Clearinghouse.

Critics wonder if users will realize that copies of their data will be stored by virtually every application that connects into Fire Eagle as well, making it extremely difficult for anyone to completely erase their tracks.

Telecom expert James Middleton wonders whether the attraction of location-based services may be overhyped, pointing out that people who go to a restaurant generally won't wait until they are standing on an unfamiliar street corner to decide where to eat.

"The industry has been wandering around in circles looking for killer services and applications that might not exist," he said. "As the joke goes, a really useful LBS application would be one that could point you to a really useful LBS application."