I'm not too pleased with the "you know it's a myth" angle… after all, the religious believer doesn't know what they believe is a myth! Make no mistake, I object neither to offending people nor to confronting them (I fully approve of PZ's 'cracker' protest), but I do object to condescension, and I just don't see how this will be productive. Personally, I find it incredibly irritating when believers presume to know me better than I know myself (such as telling me I have a god-shaped hole in my heart), and I'd like to think we're better than just turning that back on the believers. If the point of the billboards is that the reader may be an atheist and that there are other atheists out there, there are more effective ways of communicating that. If the point is to stimulate discussion, there are more productive ways of doing that, too. What I do see these billboards doing effectively is saying that atheists are proselytising jerks, and I don't see why we would want to say that.