"Wait, did.. did it just say on that bus that there's.. there's probably no.."
"You're not going to believe this, but.. probably no god."
"No god? Huh?"
"I don't know, maybe.. maybe they did a research or something. Oh god! Oh.. oh god, I can't even say oh god any more!"
"Why, WHY would they tell us like this?"
"On a bus! this is the worst day ever."
In terms of public information, I think this whole bus thing is probably attempting a tough sell. It's not the gentle reminder that most public health campaigns go for (perhaps you've forgotten that smoking gives you cancer?). Instead it conjures two images: a believer going all "well that's the point in belief/faith, dogg." And Dawkins, in an easy chair, his wool trousers riding up over his socks as he crosses his legs ladystyle, peering at you over his reading glasses, and with a smug grin: "there's probably no god, you know."
It's not going to change anyone's mind. It's designed to make those people who've already made their decision feel like some kind of cohesive group. Here's an ad for you, atheists! It's our turn now! The problem being that unlike a religon, atheism doesn't really have any core moral values or owt, other than not believing in god. (Unless you decide to go the whole hog and pay a tithe - sorry, donation - to Dawkins' little cult - sorry, organisation - whereupon he'll give you one-to-one life coaching sessions and possibly touch you where you wee).
The tl;dr version: atheist ads. Smug, silly? Effective? Or just a conversation starter?