Now, this may be considered "spoiler material" so if you don't want to read this until after you've seen the DVD and heard George Lucas' commentary, go back now.
While I was watching the film the other day with the commentary track turned on, during the battle on Mustafar George Lucas says something to the effect of "Obi-Wan turns to leave, telling Anakin 'You were the chosen one!! You were supposed to defeat the Sith, not become one!!' and he leaves Anakin for dead".
Now, if Obi-Wan thinks Anakin is dead at that point, how does he know that Anakin becomes the Darth Vader that we all know and love? I mean, how does Obi-Wan even know that Palpatine re-named Anakin Darth Vader? He never tells him that, he never hears him called that - how does he know later on?