is Threepio dim-witted or just programmed for logic? cos there are plenty of moments in the films where he simply doesn't fully understand human behaviour.
I've always considered the expanded radio plays of the original trilogy to be somewhat canon, in that they don't contradict events in the movies, but I'm pretty sure threepios's memory is erased of the princess and the Tantive's mission (?) although I'd have to dig them out and listen to them to be sure.