A dangerous precedent; someone could get a pretty nasty paper cut.

OK, so the Congress passes a law that says, specifically, that the federal government cannot open your mail without first gaining court approval.
President Dubya signs it.
And then, President Dubya issues a "signing statement" that claims he reserves the right to open mail.
Let's see, not only does he violate the Constitution's notion of separation of powers, he also violates, oh, several amendments and the basic rights this nation was founded upon.
Dubya is saying he can read your mail.
Where's it end?
All I want to say is, he who opens the Comcast bill pays the Comcast bill.