Johannus said:

To me, the most convincing way that has been suggested so far is to have Obi Wan train Anakin, then to have him train someone else. Anakin slowly and secretly begins to turn to the dark side because he thinks it is necessary to 'interfer' more than the jedi allow. Whereas, the other apprentice seems to take a more obvious slide down to the dark side. Then there would come a point where Anakin and this apprentice fight, they would both fall somewhere. Then by the time Obi Wan gets there all we see is one person putting on a helmet (which we recognise as Vader) but we don't know which apprentice it is. Obi Wan would ask about his apprentice and Vader would reply that 'he is no more'. This way we maintain the secret (to some degree), Obi Wan learns the truth, they fight, etc.

If this was the case, during all of ANH and most of ESB the audience would be thinking "That sure was ambiguous. I wonder if it's Anakin or the other guy." When Vader says "No, I am your father." the reaction wouldn't be shock or surpirse but more likely "Yeah... that makes sense." unless Anakin's fall is completley off camera, then they would likely think "What? That doesn't make sense! We need a second PT to explain this."