I was listening to some of the ESB commentary today, and GL mentions something very interesting about Vader's plan after RotS. According to him, Vader planned for a long, long time to one day kill Palpatine and become the ruler himself, but was constrained by his suit and his injuries, meaning he wasn't as powerful as Palpatine any more. But then in ANH, when Vader senses that Luke is strong in the Force and begins to realise that this is his son, George says something about Luke being "Vader's new hope" - I think this adds great ambiguity to the title of Ep IV, as Luke is not only Obi-Wan's new hope and the Alliance's new hope, but he's also Vader's new hope, at training a young, fit Sith apprentice to kill Palpatine. He talks about this during the carbon freezing chamber duel, if I remember right. It also says to me that Vader lying to Palpatine in the new scene is totally intentional, and not a continuity blip - Vader knows what he's doing right from the end of ANH, he knows that the son of the Chosen One is very powerful, and the last thing he wants is for Palpatine to find out about him.