Loopt, a Silicon Valley company that lets people interact on their cell-phones as they move around on a map, will now be available on Sprint.

Loopt lets users chat with each other and inform them of their moves around town — while bar-hopping for example — without having to send messages each time they move. Loopt automatically updates your location every 15 minutes, using GPS technology. Friends also using Loopt can check your location when they want. Other services require manual input of moves, and create quite a bit of spam because your friends get blasted by SMS updates each time you register a move. That’s likely one reason Google’s Dodgeball didn’t work out (see our comparison coverage from a year ago).

You can send messages that show your location. You can also share geo-tagged photos. The move is significant for the young Loopt, because until now it has been available only on Boost, a relatively small carrier owned by Sprint.

Cutting deals with carriers is excruciatingly painful. As an oligarchy, the major carriers protect their networks fiercely and are aggressive in looking for way to get paid. Loopt Chief Executive and founder Sam Altman told us nine months ago he was working on this and other carrier deals. He says other deals are still coming. The company says it has more than 100,000 users, though it isn’t clear how many of them are active. Sprint users will have to pay $2.99 a month for the service, another barrier to widespread adoption. Sprint would have been been wiser to offer Loopt for free, and cut some sort of revenue share on advertising. The service will be available on 25 different phones.

Former FCC Chairman Reed Hundt yesterday wrote a column for VentureBeat talking about the state of the wireless industry, and why the latest plans by FCC Chairman Kevin Martin to open up a part of the wireless spectrum aren’t good enough, and will keep power firmly in the hands of incumbents like Sprint.

Loopt is backed with $5 million from Sequoia Capital and New Enterprise Associates. Verizon Wireless and AT&T are believed to have similar location-based social network products in the works.