The government is reading your mail concerns a signing statement perpetrated by Pres Bush along with the Postal Accountability and Enhancement Act. I haven't read the act but it's apparently a general, across the board, enactment of changes that cover a broad range of issues. At issue is one section of the President's Statement on H.R. 6407, the "Postal Accountability and Enhancement Act". A section which deals with the opening of first class mail.
Back in May 2005 the administration attempted to Expand the "PATRIOT" act and beef up the governments powers to snoop on first class mail. First class mail is the regular stamped envelope mail that we used to use to send letters to one another (before E-Mail).
There is a long-standing power the government has to record "mail covers". The mail cover is the outside of the envelope, things like names and addresses, etc. The government has long had the power to record this information, effectively to take a picture of every piece of mail going through the post office. And if you ponder it for a moment, clearly that much information can be of use to investigators to establish links between individuals.
The signing statement includes this paragraph:
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.. the President is now asserting a new power, a power to open mail. The justifications are for hazardous materials inspection or to protect human life, noble causes no doubt, but also include "foreign intelligence collection". This is the administration who is already illegally wiretapping U.S. citizens and who knows what other illegal spying they're doing.