More readers have made this point:
Jonah, don’t feel bad. That whole “Darth is just a title” nonsense is just historical revisionism that Lucas came up with when making Episodes I through III. If “Darth” meant “Lord” in Episodes IV through VI (the three “true” Star Wars movies), why was “Vader called “Lord Vader” as a sign of respect instead of ”Darth Vader”? As your e-mailer said, Obi Wan Kenobi called Lord Vader by his first name Darth in Episode IV (“A New Hope,” aka the “original” “Star Wars”) in one of the finest bits of dialogue in the original trilogy: Darth Vader: I’ve been waiting for you, Obi-Wan. We meet again, at last. The circle is now complete. When I met you I was but the learner. Now, *I* am the master.
Obi-Wan: Only a master of evil, Darth.
[lightsabers clash] So feel free to call Vader by the first name Darth, just don’t do it to his face