McGregor argued that it would be pointless to try and do an impression of Sir Alec because people don't sound the same when they are young, while that is true there is still a signature in the voice that never changes and if he was trying to attempt that it failed.

He should have played the role straight without worrying about what Sir Alec did or he should have bitten the bullet and given the voice more of a Sir Alec inflection.

Sitting on the fence resulted in a very stilted performance that doesn't really pick up until he forgets about trying in ROTS.

The other daft move (or is it Darthed) was giving Anakin an American accent when he clearly hasn't got one in ROTJ.

Accents can drift completely over time (just listen to Rick McCallum) but there has to be a key for that drift (talking to a lot of people with the same accent and needing to fit in usually causes it).

If that was going to happen it would have happened by the time we saw Anakin in AOTC (being with Obi-Wan most of the time he would probably drift towards his accent which would fit in with Seb Shaw).

As Vader, Anakin doesn't have to alter his voice, the machinery does that for him, nor would he feel the need to impress people with the way he spoke (the way he choked did that).

I can't imagine his voice changing under the mask or him faking it for his last words to Luke.

I think Lucas gave Anakin an American accent to appeal more to the American market as it makes very little sense in terms of retroactive character development (almost a case of the writer becoming the character).