AT&T on Monday launched a voluntary ad system that uses the GPS positioning of modern smartphones to give deals. ShopAlerts rely on creating a GPS-aware perimeter around retail stores that will send off a text message promoting a deal whenever the phone owner passes nearby. Companies such as HP, Kmart, JetBlue, SC Johnson, Kibbles ën Bits, and Natureís Recipe are all vowing exclusive discounts for those in the right place at the right time.