Shit, I had a 2nd post that never appeared...oh well
There are mainstream keywords that bring insane revenues for PPCs. I was showing some examples yesterday to a friend and he couldn't believe in his eyes. The whole revenues from all adult keywords together can be the sum of just few mainstream ones. The moment adult keywords will be more trouble than good it's the moment they'll kill them.
Regarding google now there's an obvious shift regarding certain keywords that can be interpreted both as adult and non-adult. You won't find many adult (if any) listings high ranked:
for example:
http://www.google.com/search?hl=en&ie=UTF-8&q=hardcore
http://www.google.com/search?num=100...ex&btnG=Search
http://www.google.com/search?num=100...coff=1&q=pussy (pussy.org appears 7th and it's like the only adult)
http://www.google.com/search?num=100...off=1&q=fetish
and so on