Great thread, I've always felt that nudes or any part of the body can be considered a portrait. Even a picture of someone's hands, I would consider a portrait of the person. Let's say at an extreme, a picture of old hands, slightly curled due to pain or some type of deformity, can be a portrait of my grandmother. I think once the picture is about something beyond the person, then it may fall from being a portrait. If the pic were meant to be obscene, and the obscenity is more important than the model, then it may no longer qualify as a portrait. Unless ofcourse, being obscene is the person's personality.
I have someone interested in posing nude just to have for herself, ofcourse I get to take pictures to use for my own portfolio. I would definitely consider them portraits as I always try to stress the person's personality through the work. Just my opinion ofcourse.