Handman said:
Why call him Lord Vader when Darth is the proper title? It'd be like calling a King a Prince.
Well, people can have more than one title. Charles, Prince of Wales is also Duke of Rothesay and Duke of Cornwall. In various settings it may be appropriate to address him as Prince, Duke, or His Royal Highness.
Vader could easily have multiple titles and the proper etiquette of such things is never explained in the films so we have no reason to assume that one is a name and the other is a title.
As one possible example, Darth could be some sort of Jedi/dark side/sith title (basically some sort of title associated with evil force users) while Lord could be an Imperial/government type title. This might explain why Obiwan uses Darth while imperial officers often call him Lord. And when Obiwan does call him Darth it could almost be taken as a mocking/sarcastic use of the title implying Obiwan's disgust at his evilness/dark slide alliance.
Again, that's just one possible explanation. Personally I don't care much whether Darth is his name or title. It's just that the arguments used by people who insist that Darth was "clearly" supposed to be his name originally are almost invariably weak and easily dismissed. I have yet to see any valid argument that clearly proves Darth was definately his name and not a title in the original film. The evidence presented within the OT is ambiguous at best and Darth can make sense logically as either a title or a name.