According to a report on Times Online, the ad campaign backed by Richard Dawkins and the British Humanist Association has been reported to the Advertising Standards Authority. The claim is that the slogan violates the code that states:
[M]arketers must hold documentary evidence to prove all claims, whether direct or implied, that are capable of objective substantiation.
National director of Christian Voice, Stephen Green, say that the slogan “there’s probably no God. Now stop worrying and enjoy your life” is
…given as a statement of fact and that means it must be capable of substantiation if it is not to break the rules. There is plenty of evidence for God, from people’s personal experience, to the complexity, interdependence, beauty and design of the natural world.
Green misses the mark in what he had to say. Those things that he points to do not qualify as "evidence" in the "real" world.