As a positive atheist I'd like to answer a couple of points:
1. 'Probable' does not depict any moral uncertainty: it is a perfectly correct scientific convention. Unlike religion which rests purely on immutable dogma, all scientific principles, however well and long established, are provisional, accepted unless and until a better or conflicting theory comes along.
Thus it is right to say 'Electricity probably comprises a flow of electrons' and 'The earth probably orbits the sun'. And negatively, 'Santa Claus probably doesn't exist' and 'There are probably no fairies at the bottom of my garden'.
Similarly 'There is probably no God' is correct, even to someone who has no doubts about the matter.
2. I agree with naomi that neither atheists nor believers will be moved by these ads. But there are a lot of closet atheists out there who are only just beginning to realise that atheism is now socially acceptable; they won't be tortured, burnt at the stake or treated as pariahs any more. They are now 'coming out' as homosexuals have done in recent years. These ads, providing they don't go on long enough to become irritating, will be an encouragement to them, and, at long last, the imbalance that has existed for millennia will start that long haul towards balance.
What can be wrong with advertising truth after so many years of falsehood from the Church?