I watched Star Wars, Episode IV: A New Hope for the first time in a while a few days ago (the first step in taking my Daughter through the Machete Order). One thing that stood out was the pre-duel smack talk between Vader and Obi-Wan:
Vader: When I left you, I was but the learner; now, I am the master.
Obi-Wan: Only a master of evil, Darth.
From memory, this is the only example I can think of where 'Darth' is used as if is was Vader's first name, as opposed to the title that we (eventually) learn that it is, once we meet other Darths.
I wonder when Lucas decided that Darth would be a title for dark side Jedi (not called 'Sith' until the prequel trilogy)? In Empire and Jedi, Darth Sidious is referred to only as 'The Emperor', but 'Darth Vader' ceases to be called Darth if only one word is used; e.g. Yoda's "Vader, you must face Vader".
Is this just an example of Lucas making stuff up as he went along, rather than having it all planned out from the start (not actually all that unreasonable)? Or is there some reason that Obi-Wan used 'Darth' instead of 'Vader'? On previous viewings (i.e. before the prequels existed) I'd interpreted this as Obi-Wan being informal with his old student by using his first name, rather than his last. However, the reverse interpretation could be true once you know that Darth is a title, i.e. that Obi-Wan is using his title rather than his name in order to be as cold and impersonal as possible (i.e. replace 'Darth' with 'Sir' and put Bastard swords in their hands instead of light sabers).