President Bush's Signing statement on HR 6407, PL 109-435 Postal Accountability and Enhancement Act.
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.
Signed 20 December 2006
That doesn't say that this is already occuring, but that the President thinks he has the authority to open mail without a warrant under provisions of Title 39 and the Patriot Act.
I don't think that this line-item veto will pass Constitutional muster.