None of this is really new, but I just looked out of the window, saw a cab equipped with an electronic billboard displaying the Celtics-Wizards score (116-108) and thought this had to be included. So these mobile electronic billboards (pictured on the left) are made by Adapt Media and have been around since early 2001. The service is called Ad Runner, uses "embedded GPS, ad server, and wireless web technologies to enable electronic taxi-top billboards to receive and display messages based on the exact time and a cab's exact location" (m-pulse).
The other service (smaller picture on the right) is called Interactive Taxi by Targeted Media Partners. The service pairs GPS units already installed in the cabs with Internet-based and location-sensitive information. "The information appears on display units mounted behind the driver's seat in the cab. It is updated regularly on servers in Interactive Taxi's offices in the various cities and is downloaded to the units in the cabs" (eWeek).
Closed Spaces: Taxi