On the slight chance that you haven't already seen it anywhere else today, this is the Atheist Bus Campaign. In the words of the Guardian's Ariane Sherine:
"The atheist bus campaign launches today thanks to Comment is free readers. Because of your enthusiastic response to the idea of a reassuring God-free advert being used to counter religious advertising, the slogan "There's probably no God. Now stop worrying and enjoy your life" could now become an ad campaign on London buses – and leading secularists have jumped on board to help us raise the money."
The campaign has already quintupled its target but hell, I've always wanted someone to do this so I've punted them a few pennies and encourage any readers who walk in the light of reason to do the same.