The thing most of you need to keep in mind is that Two Face is not a cut and dried villain. He's torn between good and evil. The very fact that it's hard to picture Cusack as a villain makes him a viable Harvey Dent. One half scarred and evil, the other ... well, you get the point.
Now I'm not saying that I'm all for Cusack getting the role. I'm saying that, unlike Singer, Nolan has proven himself and I trust his judgement. If it's Cusack, then I'm fine with it. If Nolan picks someone else .. I'm fine with that, too.