Google filed for patent last month describing a technology that powers electronic billboards and displays contextual info on local merchants:
"Systems and methods for allocating space for advertisements in a network of electronic display devices are provided. Attribute information indicating retailer and categories of products available for purchase in the vicinity of a display device is maintained for each device in a database.
Advertisers may upload advertisement messages to a server specifying information such as budget, price per impression, preferred billboards and/or other constraints. One or more keywords or other descriptors are specified for each advertisement message. The system then generates an advertising campaign specifying where the advertisement message is to be output and send the messages to the specified displays. The output may consist of various forms including video, audio, printed incentive, interactive data transfers and/or combinations of these."
-- via New Scientist, Seeking Alpha