As a direct response to religious billboards, it's nice to see that atheists A: actually got a message out there, and B: did it in an unoffensive, pleasant way. On the other hand, one of my favorite billboards ever was in a picture sent by one of my brother's friends who lives somewhere in the south. In white on black, it simply said:
Go To Church
Or the devil will get you!
I'm not sure if I was laughing with them or at them (probably a bit of both,) but it really had me laughing.
This is the only billboard I've ever seen that bothered me, which shouldn't really surprise anyone on TFP who is familiar with my posting.
http://www.gunguys.com/images/posted/billboard.jpg