I forgot where this was discussed, I can't find the original mention, but anyway, somewhere someone was wondering how R2 could know how to find obi and whatnot if he had a memory wipe.
Or something, for some reason it was suggested that he must've had one because he didn't remember being on tatooine.
Grr, wish I could remember exactly what was said.
ANYWAY.
The point is, I'm staring at ANH right now and it's rather obvious, threepio doesn't remember because he probably did get wiped regularly (which is also why he doens't really have much personality). However, artoo somehow avoided said memory wipes, he remembers fine. I think he lies, LOL, but he remembers. That's how he knew to find the nearby settlements, remember when threepio asks "how do you know settlements are that way?". And how he knew where to go to find obi. And all that fun stuff.
If this is stupid and redundant, ignore me.