The Freedom From Religion Foundation will be putting their messages on 75 buses in San Francisco over the next month.
In addition to the exterior ads, there will also be 200 interior bus signs.
Considering the city already has relatively few believers, this almost seems like overkill. But you never know who may be helped by seeing a message that meshes with their personal beliefs — perhaps the ads will let someone else know they are not alone in their atheism.
(Thanks to Cale for the link!)