I've long suspected that bridges are going to be a real target. Remember Osama's quote that they would "strike at the joints"? IMO, they better be keeping a close eye on the Mississippi River bridges too.
Makes sense. Think of the economic damage, with low probability of getting caught, as there are usually only whinos hanging aroung the footings of bridges. They could concievably do dozens at the same time accross the country. This jives with bizarre reports of explosives, people found around bridges in recent months.
I think the bridges of New York are the most likely targets. The Brooklyn for its name. The Triboro and the LI Expressway bridges would really screw up NY.