The whole, "why doesent Ben recognise R2" is something that always crops up. But in that scene, the acting by Alec Guiness seems to knowingly half smile at him. His first, 'half truth' in a way. Sure he didnt own a droid, but he certainly knew him etc. One plot hole I was disappointed with, although I have only just realised it is.... "I havent gone by the name of Obi-Wan since before you were born" I know there is a deleted scene for Revenge where Obi-Wan is carrying Yoda like a baby to get back into the Jedi temple. Would have been nice to include that and have Obi-Wan saying his name is Ben whilst the guy checks the papers.