I agree with not showing anakin becoming vader. that would preserve the surprise in empire. it would also mean that alot of long hours need to go into thinking things through about how to do the secondary apprentice, maybe one given to ben for training during the clone wars.
On a side note, i did suggest in a topic a while back that palpy cloned anakin and that the clone was the dark side user until anakin duels him. Ben would catch the tail end of the duel and think that anakin was killed by his clone, thus preserving the idea of what ben says in ANH. but that was too much of a cop out and un-original. I also believe that anakin's turning needs to happen earlier than episode 3. He should turn at the end of episode 2 by being forced to defend palpy from mace windu. there's more to that statement that I have to work out but take it as you will for now.