For me, "Obi-Wan" has always been his real name, and "Ben" was only his "hiding-on-Tatooine" name. But it's just my point of view I understand the fact that he's credited has Ben "Obi-Wan" Kenobi, but I though that it was done like that just cause, in the movie, we're on Luke's point of view, and the character himself is introduced (physical introduction, I mean) as "Ben".