Having become a bit more intrigued by this thread (and less intrigued with the notion of actually doing my work, which is tedious and boring today), I checked out
http://en.wikipedia.org/wiki/MPAA_film_rating_system to review the history of the MPAA rating system, in particular, to find out when it started. Turns out that it was 1968. I then looked up films released in 1968 to suss out the first R rated film I was likely to have seen.
Turns out that it was, in all likelihood, Rosemary's Baby.