Warning: this article may be hazardous to your peace of mind because you will be reminded that you are living in a country that increasingly resembles a police state.
The most serious kind of subpoena - called a 'National Security Letter' - used to have a lifetime gag-order automatically attached. That is until Nicholas Merrill appealed his and won the right to talk about it. Despite 50,000 national security letters a year there are only three organizations who have ever won the right to say they got one. Nick Merrill explains why he's the exception and the rule.See Merrill's website, The Calyx Institute, for more details. Also, ACLU's website for their report on this case.
...as OTM notes, 50,000 NSLs and their accompanying gag orders are still being served by the government every year. And last summer, the Obama administration proposed expanding — yes expanding! — the NSL statute to allow the FBI to get even more electronic records without court approval or even suspicion of wrongdoing.(For more information about preventing government agencies and others from accessing your use of the internet, there are VPNs that can provide this security. I have little knowledge of these methods, but if you would like to explore further, this website, vpnMentor, has four pages of posts that should be able to answer your questions.)