Advertising outside of airplanes

As you know, I've been traveling quite a bit with my new job.  Flying, being in airports, pulling around luggage and the scheduling of flights is just something I enjoy.  Don't know why; I just do.

Leaving D/FW last week made me wonder about all of those buildings that surround the airport.  Why haven't they sold the roof tops to advertisers?

Think about it...

• there is plenty of space within a few miles of the airport for a message
• how many millions fly in and out of major airports each year?

Could there be a business model for an aggregator that could "lease" up the roof tops surrounding major airports and sell this to advertisers? 

I know my brother's church (one of the fastest growing in the country) has their website on top of their building to attract web traffic.

Anyone know of this going on in your city?