He was going by the name of Ben Kenobi because he was hiding, and didn't want Luke to know too much about him - Luke knew him, but as Ben, not as Obi-Wan. Which dosen't make much sense, because it dosen't take too much brainpower to know that Ben Kenobi = Obi-Wan Kenobi. Although it took a while for Luke to figure it out:

"Oh, he's not dead. Not yet."

"Do you know him?"

"Of course I know him, you stupid bastard! It's me!"

Which also brings another interesting question: why didn't Owen rename Luke as Luke Lars, instead of going by the name Skywalker? I mean, Bail Organa named Leia as Leia Organa, right? Maybe because Leia had some brief contact with Vader, and if she was called Leia Skywalker, he would see something fishy there?