I would be disappointed if Anakin became Vader early in the movie, I hope it happens later. But it need not happen right at the very end. Plenty of other movies portraying the decline and fall of a character do not leave their transformation into evil until the end. They also provide a chance to explore the fallen character's new personality as well.

For fans of the 'Godfather' movies (together with Star Wars, the two greatest trilogies ever) you will no doubt be familiar with Michael Corleone's journey to the Dark Side in the first Godfather. He finally arrives at the Dark Side about three-quarters of the way into the movie, and I believe that the movie as a whole is much more powerful and effective because we then get to see the evil Michael and the trail of human wreckage he leaves behind after he has changed.

I think a similar formula would work extremely well in Episode III. Remember that the crux of the story is Anakin's descent into inhumanity, so it would be pointless to get all that out of the way early on. Equally, we also need a chance to see just what a bastard the young Darth Vader turns out to be after he assumes his new identity (we already know how wicked the older version is).

I think the final few scenes of The Godfather were great - the new, evil, heartless Michael killing off all his perceived 'enemies' and alienating his family and former friends. Who could ever forget the baptism scene, which was juxtaposed with the brutal killings of the heads of the 5 families, whilst Michael shamelessly professed his loyalty to God. Who knows, perhaps there could be a similar climactic scene in Episode III - during the baptism of Luke (since Anakin/Vader does not know of Leia), perhaps we will see Anakin and Palpatine's goons bumping off all the remaning Jedi (Samuel L should put up a pretty mean fight) whilst evil, heartless, cynical Annie still professes his belief in Good Side of The Force.

The very last scene of The Godfather was poignantly symbolic, where one of Michael's goons closes a door on his estranged wife Kaye, thus shutting her off from his new life of evil. Wouldn't it be great if the final scene of Episode III was something similar -perhaps Vader and his henchmen closing the door of a Star Destroyer on a heartbroken Padme, who realises she has lost her true love to the Dark Side.

It is conceivable that Padme could well survive Episode III. In ROTJ Leia tells Luke that she has vague memories of her 'real mother' and remembers that she was a deeply sad person. Even if Padme gets killed off, she will have to live long enough for Leia to grow to an old enough age to be able to have conscious memories of her.