There seems to be a controversy on many starwars message boards on the fact that Obi-Wan does not appear to recognize R2-D2 when luke show up with the droid on Tatooine. The most common explaination seems to be that the obi-wan who luke meets in ANH is a clone of the Obi-wan we see in TPM.
Here's my take on it:
When obi-wan met luke and r2 it's clear that luke has stumbled upon some very important evidence about his family, which obi-wan has been trying to hide him from. On obi-wans part, by saying something like "oh yeah I know this droid" he would have been in the position to explain the entire story of episode one to luke right then and there. I think there are two reasons that wasn't done. Number one, why would mr. lucas want to tell the whole story of ep1 in ep4. secondly, Obi-wan is planning on taking luke as his jedi apprentice. if luke found out the story and realized that his own father runs the empire that he hates, then luke would he starting off his jedi training with anger, and definitely would be more prone to the dark side. (idea 1999 by Leah, please don't take it without asking)