The service, announced on Tuesday, is the first on a major U.S. carrier, and it uses GPS capabilities on certain Sprint phones so that callers can share their location and transmit proximity-based messages.

Sam Altman, CEO and cofounder of Mountain View, California-based loopt, was not shy about the expected impact of this new service. "The way we communicate on the mobile phone," he said in a statement, "is about to change forever."

New Kinds of Hide-and-Seek?

What does change is that you can know where your best friends or favorite relatives are, as long as they carry their phone. The service automates the locations of everyone in a user's private network , displaying their whereabouts on a map on the phone. If you want to know when one of those friends has gotten within shouting distance, loopt sends an alert, raising the possibility of new kinds of mall-based hide-and-seek.

There might also be new forms of tag. Users can "geo-tag" locations, which means that geographical metadata can be added to pictures or video, potentially allowing a user to automatically group video clips and pictures they took near the Eiffel Tower, for instance.

Lest philandering spouses wonder if their whereabouts can now become an open book, loopt emphasized that it is 100 percent permission-based. Locations are shared only with friends in a private network and location-sharing can be switched off, for specific friends or for all of them.

Security of one's location is a concern for many teens' parents, who might wonder if their child's whereabouts could be tracked. To assuage those concerns, loopt said it works with the Family Online Safety Institute, ConnectSafely.org, Cyber Safety Advisory Board, and others, and is TRUSTe certified.

LBS at a Major Carrier

This is not the first time that either Sprint Nextel or loopt has partnered for location-based services (LBS), said Chris Hazelton, an analyst with industry research firm IDC. He said that Nextel phones have offered LBS services, targeted at vertical-market business users, and that loopt had previously partnered with reseller Boost Mobile.

But the significance of this deal, he said, "is that Sprint is a major, Tier 1 carrier," which is now adopting an LBS that had originally been used by a smaller reseller. "This recognizes that social networking ," tied to location-sensing, could become popular among certain market segments, he said.

Hazelton noted that, for teens and college students, presence-tracking has become a way of life, and location-tracking of friends could join that style. It is positioned to grow, he noted, because GPS sensors are present in all CDMA phones, which, in the U.S., include handsets from Sprint, Alltel, and Verizon. He added that business uses for presence-tracking could include continually tracking team members on a project, which could increase in popularity as young people move into the work force.

But he cautioned that "there's a large percentage of people who don't want others to constantly know where they are." He said that, although the service can be switched off, some users might question whether they want to manage their presence constantly.