I don't see any real problem with the billboard, but I don't see any real use for it either.
You don't really get many religious billboards in England since the older generations tend to be liberal Christians and the younger ones are often atheist/agnostic. Neither group particularly cares a massive amount about religion, so billboards would be largely ignored.
Still, when I see the American billboards (both the christian ones and this atheist one) I can't help but think, "right... what are you trying to sell?" they just don't influence me towards the religion... or non-religion. In fact, I suspect the atheist billboard could be more detrimental than anything. From what I've heard, atheists aren't exactly popular in America and if an atheist group (what exactly would an atheist group do or discuss anyway? "So, we're all agreed there is no god then? Good, see you all next week.") comes across as even remotely whiny, the reaction to them probably won't be a good one.