it's very likely that 3P0 has a memory wipe. if not i'm very curious how GL will handle that.
as far as R2 i'm not sure. it might be posible that in the intervening years he didn't recognize obi-wan. or, (i know this one is gonna sound goofy) maybe obi-wan is a clone by Episode IV and R2 didn't recognize him based on that somehow.
i think i read an interview article long ago with GL where he said something like the movies (then it was gonna be 9) were supposed to be told from the perspective of R2 and 3P0 because they were the only characters that would appear in all 9 (we see some of 3P0's story telling ability in ROJ). well, GL says now there will be no 7,8,9 so i don't know if that still applies. but i use that as a basis for arguing that R2 probably doesn't get a memory wipe.
another posibility for obi-wan not recognizing R2 is that aside from color, the R2 model of astromech droid is a set standard. when you see one, you've seen them all (sorry for the cliche).