I am responding to my own post.
I just read the other posts on the subject and now have the answer. (I think) It appears doing this from a someone else's photograph is plagarism. I expected this, although I'm not sure how it is legally handled if you do not offer the portrait for sale. I do believe it could be of benefit to have one in a portfolio as no matter how many portraits I have done, if my prospective customer doesn't know any of the people, they don't really know how well the portrait resembles the real person. With a celeb, they know how they look and could get (in my opinion) a better idea of skill level. (Just my thoughts on the subject).
Thanks,
Dj