Here is what may be the official plot synopsis of the Batman Begins sequel: The Dark Knight:
"In The Dark Knight, Batman raises the stakes in his war on crime. With the help of Lieutenant Jim Gordon and District Attorney Harvey Dent, Batman sets out to dismantle the remaining criminal organizations that plague the city streets. The partnership proves to be effective, but they soon find themselves prey to a reign of chaos unleashed by a rising criminal mastermind known to the terrified citizens of Gotham as The Joker."
That is according to Batman on Film, which usually is very accurate with it's info.
Also, if you go to the newly unveiled official movie website and click on the logo, you'll see a campaign poster for Harvey Dent as District Attorney.