saw your star wars question on the forum - my guess was Obiwan didn't remember R2 because R2 wasn't his and he'd known many Astromech droids in his career (the said "I can't remember owning a droid" , or something close to that) - as for C3P0 the end of Episode 3 orders were given to have his memory erased - a common occurence with droids in Star Wars to keep them reliable and not develope erratic personalities (like R2-D2's, though his independence was often a plus)