AT&T has announced ShopAlerts, a new location-based service featuring ads and coupons from major retailers which are sent directly to subscribers' mobile phones. Initially, the service will be available in New York, Los Angeles, Chicago and San Francisco with sponsors including HP, Kmart, JetBlue, SC Johnson, Kibbles 'n Bits, Nature's Recipe and the National Milk Mustache "got milk?" Campaign.

What makes this new, opt-in service unique is that the alerts are only sent to users' devices when they are near a participating store or brand.

These geo-targeted ads are made possible thanks to something called "geo-fencing" - an emerging technology which sets a virtual perimeter around a particular geographic region.

ShopAlerts, which launched last March from Placecast, announced a partnership with location provider Location Labs, in August. It uses Location Labs and services from other partners to locate the phones in the U.S. in order to send out alerts when creating campaigns for brands. When used by carriers like AT&T and O2, the carriers themselves provide the phones' locations. Placecast manages the creation of geofences in both cases, however.

Not Another Check-in App

The difference between ShopAlerts and so-called "checkin" apps like Foursquare and Shopkick, is that you don't have to launch an app on your smartphone and take any action in order to receive the coupons a retailer or brand wants you to see. Instead, you opt in one time - in this case, via AT&T's website - and then the coupons come directly to your phone when you're in the vicinity of a store offering the deal or discount.

While this sort of "set it and forget service" may not have the geek chic appeal of a location-based checkin app like Foursqure, it has a larger potential customer base - all phones that accept SMS text messages, not just phones capable of running apps.

Placecast's ShopAlerts service has previously powered campaigns for U.S. retailers American Eagle Outfitters and the North Face and it has licensed its technology to European carrier O2 in a deal similar to AT&T's. However, this is the first time a carrier in the U.S. has offered a large-scale geo-targeted service such as this.

According to AT&T's website, the carrier expects to add more brands in the coming months, but there's no word yet on how long before the service expands beyond the initial pilot cities.