Another oversight in ROTS, because clearly in the final scene with Yoda and Obi-Wan, he's referred to as Obi-Wan, and Ben never comes up.
I agree with what was said above. Ben should have been his birth name with Obi-Wan being a Jedi Title of sorts. Another missed opportunity on Lucas part. But, hey, at least we got a wacky alien sidekick in Episode 1!