Rosalind - 3:28 pm on Dec 8, 2013 (gmt 0)
I think I recall Brett posting something years ago on this forum, to the effect that everything in the SERPs is advertising. The grey area is huge.
What about an author writing about writing, with some reference to their books? What about a politician or a political campaigner putting forward a certain point of view? Or a charity or local group talking about an event? All of those could be considered advertising, and the payment might not be explicit but that doesn't mean the host webmaster isn't getting something out of it.