In ANH we all know Obi-Wan says everything from a certain point of view. Not necessarilly lying and not necesarrilly telling the truth.
In PM we all saw Obi-Wan around R2 and now some of us know he's around R2 in AOTC.
We all figure he was probably pretending not to recognizie him in ANH until he new what was going on. We all heard him claim to have never owned a droid before. Does the Astromech in his Jedi Starfighter not count?
We now see in AOTC that Threepio now belongs to the Lars family, and R2 shows up w/ Anikin and Padme. Uncle Owen sure didn't recognize them when he bought them from the Jawas in ANH.
I understand there's many Protocol and Astro droids out there, and Threepio looks a lot different, but you would think Owen would remember someone like Threepio. Maybe he did. That's why he insisted Luke take them to Anchorhead and have their memory erased.
We all figure the droids didn't recognize anybody because of memory erases.