AT&T has announced a new location-based marketing service that delivers ads to mobile subscribers when they are near a participating brand or store. "ShopAlerts" is opt-in only, so you shouldn't receive any annoying messages unless you request them. The service is currently available to customers in New York, Los Angeles, Chicago, and San Francisco.

Supposedly the first of its kind, ShopAlerts provides customers with "valuable offers, rewards and coupons" based on their specific geographic location. AT&T says the service works by creating a virtual perimeter around a retail location, event or any other geographic area. Upon entering that "geo-fence," customers receive relevant marketing messages.

Those messages won't be limited to promotional material though, as AT&T plans to include information about the weather, traffic, and the local shopping area. At least seven companies are currently backing the service, including HP, Kmart, JetBlue, SC Johnson, Kibbles 'n Bits, Natures Recipe, and the National Milk Mustache "got milk?" Campaign.

"We are proud to take mobile marketing into the future with this unique offering that is being embraced by consumers and brands alike," said Greg McCastle of AT&T Advanced Ad Solutions. "Kmart has embraced technology to enhance our customer's experience and maintain our value proposition," said Kmart chief marketing officer Mark Snyder.