kmar, I'm trying to recall my copyright law, but IIRC, as long as the trademark owners have been making good faith efforts to protect their trademark, the courts will hold that they still retain rights to it. And that, of course, is the reason for the C&D letters--documentation of a good faith attempt to protect their trademark.
What has made this so difficult is the explosion of possible sites of violations on the internet. Before the net, violations were limited by the number of broadcast channels available, movies produced or books published each year, etc. Because the problem has grown so exponentially, I suspect that the courts will give trademark owners a great deal of latitude, regardless of how long other groups have used the registered trademarks--as long as the owners can show proof that they have been attempting to protect their trademark.
Because of the need to actively protect the trademark, I don't know that a disclaimer will do any good. In effect, it would be stating that we are using their protected trademark because the rating system has passed into public domain (which is what we mean by everyone knowing what the rating system means). It's an argument that uses as its premise part of what we would have to prove--that MPAA no longer has exclusive rights to use of the rating system because everyone uses it without referencing MPAA's creation or ownership of it.
