I think the title has more importance than just describing Batman himself.
Perhaps "The Dark Knight" refers to a dark NIGHT in the movie. Maybe there's a scene where Rachel or someone gets murdered?? Thus, that event would make it a dark NIGHT in Gotham.
And you know, Batman was never officially called the Dark Knight in Batman Begins. Maybe this movie will establish him by that name? Maybe the dark NIGHT that occurs in the movie will mark the same night that Batman becomes DARKER, and the night that the event occurs, he becomes the DARK KNIGHT, on that DARK NIGHT.
Or, whatever.
I think a title like "The Dark knight," WITHOUT Batmans name in it, tells me that it has WAAAAY more significance than just "another nickname for Batman." It has to describe an EVENT, in my eyes.