Outdoor advertising has an appeal because it is usually (except Times Square and similar) not as cluttered of a channel as TV, print and the internet. Still, I think that society has trained us to ignore traditional signs and billboards. Here are a few popular examples of those that would capture the attention of passer-bys.

This one actually has some social good, a rarity with most advertising efforts, although we are making strides (or walking in place) towards "sustainability".

Cingular made the concept of a dropped call very literal here and made the concept seem emotionally serious. I would not want to have that call fall on my head, that's why I use Cingular (and their merger with the AT&T I've had for a while).

Keep your eyes on the prize,
Dmitriy