Just minutes ago, a friend of mine showed me this blog post: http://thomashawk.com/2008/08/simon-blint-director-of-visitor.html
Photography, as an art, is protected by the first amendment. Yet, like censorship of Catcher in the Rye, even benign photography is challenged and censored. No, I am not talking about explicit pictures; I am talking about the photographer's right to make art, much like the citizen's right to speak. Photography is more controversial because it depicts things that actually exist. Most other traditional artwork depicts things that the artist imagined. Even portraits like the Mona Lisa do not entirely depict the scene in the original context, which is why photography arouses so much controversy.
A common objection we hear when we snap a photograph in a public place is that people feel that their privacy is violated when they are shot. However, if you go into any public place, anyone can see you. Same applies to photography; you renounce your privacy whenever you go out into a public place. Only when you are in the privacy of your own home, or when you are in a place where privacy is expected (e.g. bathrooms and dressing-rooms) your privacy is protected.
I also hear that photography is prohibited in some places because people feel that it may be a security threat. Now, this is a rational objection if you are taking shots of Area 51 or the Oval Office, since terrorism is indeed something to be feared in those areas, but this is not for other public areas. Photographs will not harm the museum in any way, and if it were the target of a terrorist attack (Modern art haters?), intercepting it would be the job of security.