Just as "Christ" is a title, not a surname, so too with Darth. In the presence of a Sith Lord one uses the honorific Lord, as in Lord Vader; Lord Sidious; and (ahem) Lord Brawl. However when using the third person the honorific Darth is employed.

When Obi-Wan uses this term when fighting Vader in Episode IV:

... Now, I am the master.

Only a master of evil, Darth.

... this is now generally taken as Obi-Wan using the term as a taunt to Vader, throwing his conversion to the Dark Side back at him.

Only George Lucas knows for sure.