Bruce Schneier recommends The Quixotic Quest for Invulnerability: Assessing the Costs, Benefits, and Probabilities of Protecting the Homeland, which concludes amongst other things, that we abandon any effort to imagine a terrorist target list.
Maybe, but it seems likely that targets that have been hit in the past will be targeted in the future. It is difficult to imagine that our adversaries would ignore possible targets such as the White House, Capitol, and Pentagon. But then maybe he is correct. Once you start thinking of likely targets in the Greater Washington area you will never stop.