He told Gordon he was going for Rachel, so that means the Joker lied. If Batman was really going after Dent but told Gordon he was going after Rachel, wouldn't he and Gordon have ended up at the same location? The only time that the locations are mentioned is when the Joker reveals them.
Looks like it made $9,629,366 on Tuesday, which moves it to $333,929,159. That's good for 16 on the all-time domestic list. I'm thinking it'll pass both Spider-Man 3 and then Finding Nemo today, and then The Two Towers tomorrow and into the Top 10 by the end of the weekend.
This suggestion might seem off-topic, but I think it's relevent to understanding this film a little better based on some of the discussion here about motivations.... Read the graphic novel Watchmen (yeah, the one they are turning into a movie). I think it'll make some parts of the movie easy...
They were afraid that the Joker would blow open the prison releasing all the criminals, so they were sequestered on what they had assumed would be a safe ferry until the threat could be neutralized.