If you receive a national security letter, you are not allowed to tell anyone (not your family, not your lawyer, not anyone).

If you break the life-long gag order, you go to jail for up to 5 years.

http://www.wired.com/threatlevel/2010/08/nsl-gag-order-lifte...;

Hundreds of thousands of these have been issued, and the above story (about the owner of an ISP) is one of the few cases where someone fought back and won the right to speak about it.

That can't be constitutional. The government has no right to even know whether you told your lawyer or not.

At least sometimes that it is the case, though I don't know if it generally is.

If you have been authorized to have access to classified information, such as (if one accepts the authenticity of the presentation that the Guardian and WaPo have reported on) even the existence of PRISM, on the other hand, there are fairly substantial legal consequences to unauthorized divulgence of that information, and those consequences don't necessarily go away because someone else gave out the information first.