There is speculation (here and here, for example) that the NSA surveillance is the kind that doesn’t fit in with existing laws.

For example, Echelon.  Wikipedia describes Echelon as follows:

ECHELON is thought to be the largest signals intelligence and analysis network for intercepting electronic communications in history. Run by the UKUSA Community, ECHELON can capture radio and satellite communications, telephone calls, faxes and e-mails nearly anywhere in the world and includes computer automated analysis and sorting of intercepts. ECHELON is estimated to intercept up to 3 billion communications every day.

In other words, what the NSA is doing is not tapping a specific person or conversation, but essentially monitoring (electronically) billions of communications by computer simultaneously, hoping to "hear" (again, by computer) certain patterns of words or phone numbers.  Once a conversation is flagged, this begins the investigation process.  The rest of the intercepted communication essentially becomes "garbage out".

If this is indeed what is happening, then it is impossible to get an individual warrant. 

Moreover, one can only speculatae as to the extent to which the Echelon program is actually used.  If it does exist as I have described it, it certainly is a valuable tool, whose utility is (arguably) diminished once it becomes known.  Therefore (the argument goes), Bush was justified in using it, and not telling Congress about it before, and not telling the American people about it even now.

But I’m largely with Matt Yglesius on this.  Too bad.  We don’t skirt the Constitution simply because new technology enables us to:

I’m perfectly happy to believe that the decision to deploy illegal NSA wiretaps was driven by some novel technological development. I’m likewise happy to believe that the novel development in question may be a very useful tool in fighting terrorism. I’m even willing to believe that the new technology may somehow be such that public disclosure of its existence — by, for example, initiating a congressional debate about whether or not to deploy it — might have in some way compromised its utility. At that point, however, you have to say "so much the worse for new technology."

If something is invented that would be useful to deploy, but also illegal to deploy thanks to an outdated 1978-vintage statute, the thing to do is to change the statute. The view that the President just gets to do whatever is simply not going to pass muster. Presidents may well have good reason for wantng to do whatever, but you can’t run a country like that. Bill Clinton’s health plan was a good idea. In some sense, if he’d just overridden or ignored congress and put it in place, that would have been good for the country. In another, more important sense, it would have been disastrous.

As the saying goes, the constitution is not a suicide pact. But by the same token, not every risk that someone somwhere might get killed is license to kill the constitution. Laws are laws, and if they’re somehow found wanting, they ought to be amended or repealed not just wished away.

The Clinton administration program, code-named Echelon, complied with FISA. Before any conversations of U.S. persons were targeted, a FISA warrant was obtained. CIA director George Tenet testified to this before Congress on 4/12/00:

I’m here today to discuss specific issues about and allegations regarding Signals Intelligence activities and the so-called Echelon Program of the National Security Agency…

There is a rigorous regime of checks and balances which we, the Central Intelligence Agency, the National Security Agency and the FBI scrupulously adhere to whenever conversations of U.S. persons are involved, whether directly or indirectly. We do not collect against U.S. persons unless they are agents of a foreign power as that term is defined in the law. We do not target their conversations for collection in the United States unless a FISA warrant has been obtained from the FISA court by the Justice Department.

Meanwhile, the position of the Bush administration is that they can bypass the FISA court and every other court, even when they are monitoring the communications of U.S. persons. It is the difference between following the law and breaking it.