

There has been a lot of misunderstanding, all the way from the former Director of the NSA to the blogosphere, about the "standard" that must be respected regarding domestic surveillance. Those wanting the most restrictive standard talk about the "probable cause" required for a warrant to issue from a magistrate, whereas those wanting to bypass that standard talk about whether a search is "reasonable" so as not to require a warrant. We'd best start with the text of the Fourth Amendment:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
Self-education on this topic could hardly find a better starting point than Cornell Law School's annotations to the Constitution. The first one to read is "History and Scope of the Amendment." Its subsection "Scope of the Amendment" contains a history of Congress' adoption of the specific language and the reinsertion of a previously rejected substitution of “and no warrant shall issue” for “by warrants issuing.”
Madison’s introduced version provided “The rights to be secured in their persons, their houses, their papers, and their other property, from all unreasonable searches and seizures, shall not be violated by warrants issued without probable cause, supported by oath or affirmation, or not particularly describing the places to be searched, or the persons or things to be seized.”
See also footnote 11 on this point:
The amendment was originally in one clause as quoted above; it was the insertion of the defeated amendment to the language which changed the text into two clauses and arguably had the effect of extending the protection against unreasonable searches and seizures beyond the requirements imposed on the issuance of warrants. It is also possible to read the two clauses together to mean that some seizures even under warrants would be unreasonable, and this reading has indeed been effectuated in certain cases, although for independent reasons.
The text preceding footnote 11 explains:
There were, however, lawful warrantless searches, primarily searches incident to arrest, and these apparently gave rise to no disputes. Thus, the question arises whether the Fourth Amendment’s two clauses must be read together to mean that the only searches and seizures which are “reasonable” are those which meet the requirements of the second clause, that is, are pursuant to warrants issued under the prescribed safeguards, or whether the two clauses are independent, so that searches under warrant must comply with the second clause but that there are “reasonable” searches under the first clause which need not comply with the second clause.
As the annotation rightly describes:
This issue has divided the Court for some time, has seen several reversals of precedents, and is important for the resolution of many cases. It is a dispute which has run most consistently throughout the cases involving the scope of the right to search incident to arrest.
As I have written previously in You Be the Judge, however, the Supreme Court has also had to decide what constitutes a "search" in the area of electronic surveillance, finding that a warrant is required when the surveillance is pursuant to a criminal investigation but leaving open the question "[w]hether safeguards other than prior authorization by a magistrate would satisfy the Fourth Amendment in a situation involving the national security." The Court a few years later said in United States v. United States District Court (1972) that "Fourth Amendment freedoms cannot properly be guaranteed if domestic security surveillances may be conducted solely within the discretion of the Executive Branch." All serious students of this great controversy of our time should read the full opinion of the Court, if not also Justice Douglas' concurrence, which presciently described many of the issues which have arisen since George Bush's expansions of warrantless surveillance on the American people. This case is covered in more detail in the annotation "Electronic Surveillance and the Fourth Amendment" and its subsection "Warrantless 'National Security' Electronic Surveillance," but see footnotes 152 and 153 for reservations on the scope of the decision.
Since that case, however, the Court has drifted from a general "view that warrantless searches are per se unreasonable, with a few carefully prescribed exceptions," to a broader "view of permissible exceptions and of the scope of those exceptions," so that:
By 1992, it was no longer the case that the “warrants–with–narrow–exceptions” standard normally prevails over a “reasonableness” approach. Exceptions to the warrant requirement have multiplied, tending to confine application of the requirement to cases that are exclusively “criminal” in nature. And even within that core area of “criminal” cases, some exceptions have been broadened. The most important category of exception is that of administrative searches justified by “special needs beyond the normal need for law enforcement.” Under this general rubric the Court has upheld warrantless searches by administrative authorities in public schools, government offices, and prisons, and has upheld drug testing of public and transportation employees. In all of these instances the warrant and probable cause requirements are dispensed with in favor of a reasonableness standard that balances the government’s regulatory interest against the individual’s privacy interest; in all of these instances the government’s interest has been found to outweigh the individual’s.
You can read more about this process in the annotation "Valid Searches and Seizures Without Warrants."
As Secretary of Defense Rumsfeld said, "You go to war with the Army you have. They're not the Army you might want or wish to have at a later time." Correspondingly, we get decisions made about "unreasonable searches and seizures" by the President, Congress, and Supreme Court that we have. Perhaps our current Court will issue a definitive judgment if, as an earlier Court commented, a suitable case can ever make it there. As for the other two branches, if you don't like the way things are going, there are elections in 2006 and 2008. There is uncertainty in this area of constitutional law; but I hope this post eliminates some confusion about what there is to know.