If you think about Star Wars episodes 1-3 (which I tried very hard not to do, because they were traumatic enough without me applying my brain) there are some serious inconsistencies between the back story alluded to in the original films and the same story revealed in the new ones.
Via Kottke, Keith Martin finds a way to reconcile the differences. And in his version, R2D2 and Chewbacca are master spies who are ready to assassinate either Luke or Leia without mercy if they show the slightest tendency towards sithiness.
On first seeing R2, Obi-Wan has a twinkle in his eye and calls him "my little friend". Well, he is. However, when Luke wakes up and says that R2 claimed to be owned by an Obi-Wan Kenobi, he blandly says "I don't seem to remember ever owning a droid." Ben has in fact owned several but the remark is aimed at R2 and translates as "You keep quiet. I'm not about to tell him everything just yet." Obi-Wan thinks fast and tells Luke a version of his past that does not involve a father who became a dark lord of the Sith. He wants to examine Luke a lot more closely before he risks telling him the real truth.
UPDATE: Here's the link