Check out this company. It is the first company I have seen that has really figured out how to deliver location-based advertising. They blow away companies like LAT49. The later focuses on delivering ads on maps. The implementation is simple and unsophisticated. Placecast processes multiple types of location, demographic and other information and then adjusts the ad delivery based on what is relevant in terms of location. They have a robust network of advertisers. They have sophisticated engines that drive the ad delivery and they can plug and play with major players such as DoubleClick/Google.