Well, I'm thinking of making it look like Obi-Wan is scared off by the Emperor's arrival to Mustafar by moving footage of the Emperor's shuttle up to match when Obi-Wan leaves. This will give the impression that Obi-Wan had no choice to leave Anakin at the end of the movie and is aware he's probably still alive. (The Emperor will hopefully appear to be arriving to meet up with Anikan, thus negating the need to explain how he got from Coruscant to the "most-a-far" system in the outer rim in under 10 minutes). That way Obi-Wan and Yoda will both realize what has happened, and will of course eventually learn what has become of Anakin and his new name, however this would then happen between episodes III and IV keeping the critical details out of the audiences grasp. It will be hinted at in Episode IV when Obi-Wan mentions that Vader was a pupil of his, till he turned to evil, but the line "Your father was killed by a young Jedi named Darth Vader" will hint there may be more to the story than we know, or that Obi-Wan is telling. This of course is revealed in V when Vader admits to being Luke's father.