It is amazing. A viral campaign apparently already started for a movie, that will launch in 2008. That is at least 7 months down the line. I wonder how early is too early?
But, given the fact that it is a viral campaign, they need some time for spreading it.
There is a good overview about what has happened so far over here.
It seemed to have started with a static site, then some unbranded posters appeared in some US cities. Then:
Within 48 – 72 hours, all of the posters in major cities were defaced to resemble the image seen at the URL I Believe in Harvey Dent Too. If you went to that URL the site would ask for your email address. After you submitted (reusabale database anyone?) you were sent an email with the coordinates for a pixel that you could removeBatman4 from the site.
With users able to remove just one pixel each, a viral effort was underway to spread the word to get others to register and remove a pixel. In less than a day, thousands and thousands of unique visitors had visited the site to remove a pixel – ultimately revealing the face of the new Joker, Heath Ledger.