If only homeland security were this simple:

Conducter on a DC metro train:  "Passengers, please refrain from taking pictures of the train due to terrorist threats. Terrorists, please don't take pictures of the trains."

-- from DCist's weekly feature Overheard in DC, in which Washington, DC, residents submit snippets of dialog they happened to pick up (it's a feature DCist has knowingly borrowed from Overheard in New York).