New York, Atlanta, Miami (some), Denver, LA, Dallas. Those are cities I know have them. As far as my personal experience with them, I don't really pay attention. I'm not sure how much advrtisement money they could bring in. I'm sure it would be more expensive to advertise on the outside of a cab though than the in.
Best of luck though.
Enjoy Life,
Nick
When I stand before God at the end of my life, I would hope that I would not have a single bit of talent left, and could say, "I used everything you gave me."