If you go to www.ibelieveinharveydent.com you'll see a campaign ad for Harvey Dent for District attorney. Keep in mind, this is official The Dark Knight stuff.
Then, if you go to www.ibelieveinharveydenttoo.com you'll see a prank by the Joker himself. When you input your email (apparently Hotmail doesn't work), and the code that appears, you'll get an email with a link and some coordinates. Click the link, put in the coordinates, and another pixel is taken from Harvey Dent's already "defaced" picture, slowly revealing Heath Ledger as the Joker.
"I always say, you never know what a man is truly made of until you peel the skin off his face one piece at a time." -- Quote from the Joker's email