Well, as crazy as it sounds, the MPAA has begun to send C&D letters to some websites hosting fanfics that use 'their' ratings. The whole G, PG, PG-13, R and NC-17 thing.
fandom_lawyers has a very informative post here.
I honestly don't know what to think beside WTF? I guess we'll have to wait and see where it goes.
www.aestheticism.com got a letter too, and already changed their ratings.
In the fandom_lawyers comments, an explanation is proposed. It says there is a slight chance Harlequin may start using the ratings on their books.