OutDoorAdNet is a Google Maps mashup for the outdoor ad industry that brings the buying and selling of outdoor ad media online. It claims to be "the industry's first map-driven interactive website that focuses exclusively on the buying and selling of outdoor advertising." The mashup displays outdoor ad media listings pinned on an online interactive map, combined with census demographic data. Although the coverage is only for California the app shows what's possible for this vertical industry, worldwide. I suspect satellite imagery present within Google Maps might also help ad buyers with visualizing where the ad placement will be in relation to roads, highways etc..
MIT Advertising Lab: Advertising with Google Maps
Google Maps Mania (Aug.9/05): Will ads target the satellite cameras? (Part 2)
Rooftop ad examples highlighted by Google Maps:
Target Store, Pepsi Logo