Vaderisnothayden said:
But do we really know for sure that Lucas didn't originally intend Anakin to be Darth Vader? I mean, I've long theorized that he invented that bit some time after he made the first film, but I don't know.

Yeah, because in some of the early drafts Anakin and Darth Vader are different characters. There is alot of evidence to show that Lucas did not originally intend Anakin to be Darth Vader, and certianly didn't intend for Leia to be Luke's sister. All this was made up as he went along. One of the members here has written a great book called The Secret History of Star Wars, it is a great book that does some impressively extensive research into the history of the franchise that is usually ignored or over looked by most other "Making of"s. It is definitely worth reading. He used to provide a free PDF version of the book on his website, but since he is actually getting it published now, he has taken it down. Once the hard copy is availible, it will definitely be worth dishing out the money for. It was too good of a book to spend its life as nothing more than a pdf ebook. This book would really give you a lot of this information you seem really interested in.