After Vader says this line, does he then say 'if only you knew the power of the dark side?' after it? I can't remember that scene all too well at the mo. I was just thinking that if he did, then the change to 'your mother' may not make complete sense, as it would imply that Padme knew a great deal about the Force, which isn't very accurate. Wait, or is it? Lol I just remembered that in Sith she does mention the Dark Side and the Sith to Anakin on Mustafar after she was informed by Obi Wan on Courouscant.
Damn Lucas and his swampy six-film plot!!