From The Standard.com Target marketing via RFID to debut in Seattle .
Some cafes and retail stores in Seattle next week will begin individually marketing products and services to bypassers in Seattle using RFID (radio frequency identification) technology.
Users of the personalized marketing system carry an active RFID tag roughly the size of a stack of four credit cards. When the tag comes within 100 feet of a transmitter sending low frequency signals at 126 kilohertz, the tag transmits a unique identification signal to a receiver connected to a monitoring and execution server.
Substitute the "stack of four credit cards" with a cell phone. Combine an RFID tag inside (it could be a credit card or passport) with the GPS function and you get personalized advertising.