I know the Harvey Dent website (http://ibelieveinharveydent.warnerbros.com/) was linked in the other Bats thread, but AICN has uncovered some more.

Go to http://www.Ibelieveinharveydenttoo.com to see a defaced version of the campaign poster. Sign up your email address and they'll send to a pixel coordinate (mine was 560, 380). Click on the link, enter your coordinate, and that pixel will be removed from the campain poster, revealing... something. Seems to be a white face.... The more people who play, the quicker it gets uncovered.