Heh. I know that Harey Dent is Two-Face; I read The Long Halloween, you know. What, my sig makes you think otherwise?
But anyway, I don't think that he's just going to be in the third movie; Two-Face
is going to be in The Dark Knight, from what I know. Of course, I've been trying to avoid spoilers, so I wouldn't really know.
Edit:
That was an image taken from the viral marketing "I believe in Harvey Dent" campaign. Originally, Ibelieveinharveydent.com was complimented by an Ibelieveinharveydenttoo.com, which was Joker mocking Harvey Dent's run for district attorney(The site was altered, however; the image is no longer present.)