I find this incredibly laughable.
Forget that they are "offended". Of course they are. And we are offended by their ads. So what?
Here's the kicker, though.
The text of the ad reads "There is probably no God. Now stop worrying and enjoy your life."
The complainers say "It is given as a statement of fact and that means it must be capable of substantiation if it is not to break the rules [of the Advertising Authority]."
"Probably" is not a statement of fact.
Secondly, I'd like to see Christians have to prove their "statement of fact" that non-believers are going to burn in hell (which was the text of the ad that sparked this bus campaign).
If that's going to be their tactic, let's demand the same standards of "proof" from them. Let's convene a court and see if god shows up! Conduct an investigation and see if we can find testimonials about hell from people who have been there, or maybe the location of hell...
The hypocrisy is truly astonishing.