If I'm referring to the prequels, I call him Obi-Wan, but I call him Ben if I'm referring to the originals, because I learned how to talk Star Wars in the Luke Skywalker Academy of Speech. Actually, I've always found it rather jarring when Luke calls him Obi-Wan in Return of the Jedi. It just didn't seem representative of their relationship. Only the old people called him that! Does anybody else feel that way?