As already mentioned at Friendly Atheist and Dispatches from the Culture Wars, the folks NEPA Freethought Society have been pressing a legal fight with help from the ACLU to gain equal access to the buses which have previously allowed advertisements from several churches and at least one highly controversial blog.
Most any atheist organizer can tell you that we tend to get shut down in the public square. If you doubt this, just Google “atheist billboard rejected” or something along those lines for a litany of examples. The Coalition of Reason had to shop around here in OKC until they found a willing vendor, and this sort of problem isn’t exactly unique to the Bible Belt.
The great thing about a government-sponsored designated public forum is that, unlike private vendors, they cannot arbitrarily reject messages which they find distasteful. The other thing about such a forum is that it exists (or not) on account of a pattern of government behavior, merely disclaiming an official intent to create a forum is not enough to shut it down. As the ACLU contends in their complaint:
[The transit system] through its longstanding practice of accepting all advertisements for placement, including advertisements that would violate policies in place at the time they were published, created in its advertising space a designated public forum.
This raises the obvious question of whether your local municipality or county have been engaging in the same sort of content-neutral advertising which Lackawanna Transit System is alleged to have undertaken in Pennsylvania. If so, you may have an opening. Keep an eye out.