This is actually kinda
old news over here, I seem to recall it was put on the buses some time last year.
It was also funded by Christian groups as well, not just the New (read: Old) Atheists.
(A lot of people have been shocked out of their comfort zone by the ads, as it takes away from their London day-to-day trip to work and back with their blinkers on)
The American one "Why believe in a god? Just be good for goodness' sake" leaves open a massive debate about the definition of goodness within the Humanist context. (i.e., is it 'good' to abort a pregnancy that might otherwise be an 'inconvenience'?) I used to be a Humanist for a few years (even saw Dawkins/Hitchens etc. debating publically a few times against various theists, often with a feeling that something wasn't quite right), it was this whole area that made me re-think everything. I think these ads actually help people into actually thinking
about these issues for the first time in a long while.