http://www.whitehouse.gov/news/releases/2006/12/20061220-6.html

The pertinent excerpt:

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.

Even though I dislike and distrust the signing statements in general, this one doesn't seem to be a problem. The President goes out of his way to point out that the Executive Branch will be opening a lot of mail ("to the maximum extent permissible"), but apart from that, it isn't extreme in its assertions.

Mail has always been subject to inspection. The recipient is always aware that an inspection has taken place, because the envelope or package has to be opened. People have been discouraged from sending cash and uninsured valuables for that very reason.

Technically, once a person posts an item, the item is in the legal custody of the Postal Service until it's either delivered or returned to the sender. The Postal Service is well within its authority to inspect mail for harmful substances. If the Executive Branch gets a warrant -- and a close reading of the signing statement yields nothing about warrantless search -- its agents can conduct such a search as well.

Of course, the recipient or the sender can complain about opened mail, to the Postal Service, to the Executive Branch, and to the media, and demand a reason for the search. Any abuse of this practice should and must be exposed by the citizens who are affected. Also, if someone is overly worried, there's always UPS or FedEx, which are not government agencies and therefore both more and less responsive to their customers' needs and worries.