ARS Technica reports Google filed a patent application in September 2004 that describes a method of providing highly targeted advertising to users of Wi-Fi access points and passing some of the advertising revenue on to the Wireless Access Point (WAP) provider.
Ad selection under this model could be based on any combination of "geographical location of the WAP, an operation of an entity providing the WAP, selected by the entity providing the WAP, and a profile of the WAP." When you see "operation of an entity," think "the business the WAP provider is in," such as coffee shops for Starbucks, bookstores for Borders, or airports for JFK. There are plenty of examples of various selection models, including basing the advertising on individual user profiles, presenting ads for businesses located in physical proximity to the access point, or promoting services related to the business serving up your traffic. Feel free to peruse the original document for further examples.