One publisher actually was accused of "breach of contract" over applying the nofollow attribute to an advertisers link.
So I JUST got an email from the once a quarter person that I contact about payment saying that these nofollow tags need to be removed!
"This needs to be removed as per our initial agreement"
In fact, some publishers admit to cloaking the advertiser links to be nofollowed to GoogleBot and not to humans.
I've been nofollow'ing paid links for some time, although in my case I cloak to only serve nofollow to GoogleBot. Unless someone has purchased a link explicitly for SEO benefit then they cannot expect to get it.
I wonder if Google would consider this form of cloaking to be acceptable?
In any event, here is really the first discussion I have seen about these two concerns:
(1) Advertisers cloaking their paid links to be nofollowed in the eyes of the search engine spiders. (2) Publishers claiming breach of contract for nofollowing links they paid for.