>>1102IIRC, Organa's exact line when he gives the droids to Captain Antilles is "have the protocol droid's memory wiped." Unless I'm taking it too literally, I don't think R2-D2 ever got his memory wiped, and since Artoo was supposed to recount the whole saga to the Whills a century after the Battle of Endor (Lucas's idea, so canon as of the making of ROTS), he'd have to remember it somehow.
Honestly, I think the line was just Lucas covering his ass. Nothing in the OT suggests that Threepio knows Darth Vader is Anakin, much less that was
created by Anakin, so he'd have to have some kind of mind wipe between Eps III and IV for the sake of retcon. Meanwhile, Artoo, who can't communicate anything to the audience because we don't speak his "language," is free to have all the insider info he wants. Convenient.
I'm sure there's a canon explanation for this, and I sure as hell hope it's better than mine.