As you may know, photographers have been hassled for taking pictures in public places - yes, even tourists taking pictures of iconic spots. Now, according to a federal bulletin, this is to stop. See: http://www.crunchgear.com/2011/01/27/photographers-youre-now-officially-free-to-shoot-in-public-places-and-outside-federal-buildings/ and also http://www.andrewkantor.com/useful/Legal-Rights-of-Photographers.pdf Now, in reality, might you still get harassed by a security goon? Sure. So, you may wish to print the linked story(ies) out and carry it (them) with you for their edification. Note, private property is private property and you must abide by the property owner's wishes per the above articles.
Note, I am not providing legal advice - just letting you know about websites that discuss legal rights regarding photography.