In answering a fan letter about the possibility of his playing Dumbledore, Sir Ian McKellen had this to say (dated Jan 11, 2003):
Dear concerned mailer,
I have not been offered Dumbledore but another knight is considering it.
Best wishes
From the actor who plays Gandalf for God?s sake,
Ian McKellen
In case you were wondering, IMDB.com lists Michael Gambon as a knight (not that that's any kind of confirmation, or we don't have other elderly actors who have "sir" in front of their names. I'm just sayin'). Much thanks to Sir Ian McKellen's official site.