Here in New York City, there has been a campaign since 9/11 with signs and posters on buses, trains, stations, etc., that say "If you see something, say something." Last year there were posters showing pictures of eyes (about 12 pairs). The text indicated that "New York has 18 million pairs of eyes. Last year there were many reports of suspicious activity."
There are signs on all bridges prohibiting "use of a camera." But if you go into any NYC bookstore or any of the stands selling tourist items, you can find exquisitely detailed pictures of every bridge, tunnel, monument or important building in the city for $2.98 and up.
I have been stopped numerous times when shooting subjects such as, for example, the gargoyles near the top of the Chrysler building. The police thought I was sneaking terrorist pictures with my stealthy Canon 1D MkII and the very covert-looking 100-400mm lens.
In Philadelphia I was stopped while taking pictures of Independence Hall at night. I was asked what I was shooting. Then I was asked why. When I said the building was pretty at night I was asked for ID. When he saw that my wife (also taking pictures) and I were from NYC we had to explain why we came all the way to Philadelphia to take pictures at night.
Again, even if night pictures of Independence Hall would be of use to terrorists, they can get them at Amazon.com for less than it would cost them to park their car in downtown Philly.