Supposedly the line uttered on set was actually:Luke, I am your father!
While it doesn't flow quite as well this line seems to make more sense based on what we learned from the battle on Mustifar.Obi-Wan killed your father
It also flows better with Obi-wans famous point of view speech
If the conflicting view points are Vader claiming that Obi-Wan killed Anakin and Obi-Wan claiming Vader killed Anakin then the above seems to make more sense.Your father was seduced by the dark side of
the Force. He ceased to be Anakin Skywalker
and became Darth Vader. When that happened,
the good man who was your father was
destroyed. So what I have told you was
true... from a certain point of view.
Rather than Obi-Wan's view point being that Anakin was already dead when he chopped him into pieces on Mustifar it becomes a battle of shifting viewpoints on just who was responsible for the fall of Anakin Skywalker.
Comments, thoughts, ideas?
BTW Anybody know just how Vader learned Luke was his son?