It used to be that certain types of domestic postal mail were protected from warrantless search by the government, something termed a "mail cover". The protection covered "sealed" classes of mail - first class, priority mail, and express mail, and was a good and just protection against unwarranted snooping in a post-Church Committee world.
This is no longer a post-Church Committee world, though; in this post-9/11, war on terror world of George W. Bush's we live in, things are different. Very different.
In December 2006, Congress passed H.R. 6407, the "Postal Accountability and Enhancement Act". Approved by both House and Senate, part of the bill converns mail covers. In a clear-cut effort to extend, enhance, and reinforce the decades-old (post-Watergate) restrictions on mail covers, part of the bill reads "The Postal Service shall maintain one or more classes of mail for the transmission of letters sealed against inspection. The rate for each such class shall be uniform throughout the United States, its territories, and possessions. One such class shall provide for the most expeditious handling and transportation afforded mail matter by the Postal Service. No letter of such a class of domestic origin shall be opened except under authority of a search warrant authorized by law, or by an officer or employee of the Postal Service for the sole purpose of determining an address at which the letter can be delivered, or pursuant to the authorization of the addressee."
It sounds pretty clear cut. "[S]hall not be opened except under authority of a search warrant authorized by law" doesn't leave a lot of ambiguity; where the USPS is concerned, the only applicable warrants are federal ones, from a federal court. Seems reasonable to me.
George W. Bush didn't agree. Or, perhaps, he didn't have the balls not to disagree.
In his December 20th signing statement, he - or, more likely, Administration lawyers - shat out this little interpretive gem: "The executive branch shall construe subsection 404(c) of title 39, as enacted by subsection 1010(e) of the Act, which provides for opening of an item of a class of mail otherwise sealed against inspection, in a manner consistent, to the maximum extent permissible, with the need to conduct searches in exigent circumstances, such as to protect human life and safety against hazardous materials, and the need for physical searches specifically authorized by law for foreign intelligence collection."
So, essentially, undefined "exigent circumstances" (examples are provided, but they're certainly not specific nor all-inclusive) trump the law, in Bush's (administration's) eyes. Never mind that Congress' will was pretty clear in the matter; the White House has spoken. No mail is any longer safe from being opened in "exigent circumstances" - circumstances defined and identified soley by the government, as and when they wish. Gone is the expectation of privacy, of protection from "Big Brother" snooping.
If you need to communicate in privacy, "sealed mail" would seem to no longer be a viable option. Gone are the last of the low-tech means of ensuring privacy; left are largely electronic encryption programs - Phil Zimmerman's Pretty Good Privacy, or PGP, and it's intercompatible open-source counterpart GPG; TrueCrypt and other disk-encryption utilities, and similar pieces of software.
Do you remember the pretty lies the President told in the months after 9/11, about how "we" must not allow "terrorists" to erode our freedoms and destroy "our" ways of life? Do you remember the hue and cry about "activist judges", how legislation should come from the legislative branch of the government, and nowhere else? Lies and hypocrisy, dear reader, lies and hypocrisy. The Administration is creating legislation from the Oval Office, and the only terrorists I see eroding freedoms anywhere near here are not dark-skinned Moslems, but overweight white Christian men in expensive suits, liars and hypocrites all.