Just a few months ago, roof top ads suddenly got a lot of attention when people started spotting them in Google Maps and Google Earth. For example, Target has a few stores with their distinctive target-symbol logo painted on their large roofs. They did this when the roof can be clearly viewed by nearby skyscrapers or planes about to land at an airport. They have been pleasantly surprised with the much larger ad impressions they are getting.

Over the last few months, companies selling services to do roof-top advertising have been getting more attention and some new ones have been started to focus on the larger market of available viewers on roofs. Here's some articles from MIT Advertising Lab and Wired on rooftop ad companies.

The big drawback to doing this is that there is no set schedule for when new satellite photos of roofs and fields will be done. So, you could put up an ad tomorrow, but it might not be seen by a satellite or aerial photographer for months or years. And, it might be even longer before Google chooses that photo and puts it up in their database. So, expect your sky-viewable ad to be a long-term investment.