It looks like the title question is going to become the legal question of our age.
Over at Random Nuclear Strikes blog, there is an interesting compilation of thought on the subject, complete with linkage. I commented over there also.
This is something that requires more in-depth examination, because it resembles Josef Stalin's administration of the USSR, and how they skewed the definition of mental illness to incarcerate ALL the opposition to Stalin's dictatorship. Might we be heading for similar abuse of power by allowing the Government to maintain a "Terror Watch List", refusing Constitutional guarantees that should have protected us from such a list? It is a very short step to go from having this list to taking action based on it (other than refusing commercial flight).
The idea of having a "watch-list" is not new, various policing agencies have been using them forever, and in fact, they are indispensable for police work. Yes, you have to have a list of "usual suspects" to start from when trying to solve a crime, or else you have to start over from scratch with every case. I learned as police recruit that out of the hundreds of thieves resident in my jurisdiction, only 30 or so were prolific burglars, so if I lifted a latent fingerprint at a burglary investigation, it was immediately checked against that list of 30. Sometimes the forensics guys could get a positive match even before I had finished interviewing, photographing and gathering evidence on M/O, etc.
Yep, gotta have suspect lists. There's a difference between a suspect list, generated from actual arrests and convictions, and a "watch-list" generated from anonymous tips. As a cop, there was NO way I could build a Probable Cause case to get a warrant or make an arrest just on a telephoned tip. All I could do was make a note of the tip, and look into what the tipster claimed was true. Sometimes, a tip leads you to the truth, but mostly, they are dead ends which can't be used to clear a case.
Enter the Transportation Safety Administration, or TSA, part of the Homeland Security apparat. Under GW Bush, we got the Dept of Homeland Security as a Cabinet-level position (when giving more dollars to both the CIA and the FBI would have sufficed) after 9-11. Since hi-jacked aircraft were used as weapons on 9-11, we also got the odious security hassles at airports, etc.
The sleuths in Homeland Security demanded that a "Terror Watch List" be initiated, and so it was. It was probably a good idea at the time, but an idea that went bad, because once a tip from anyone puts you on the list, you can never fly commercial again. There is ZERO way to get a court hearing to get you off the list, because the Government refuses to disclose it, even to a Federal Judge. The Watchlist has been "lawyer-proofed", which is to say, in fact, it rises above the Constitution. Now, there are cries to remove a citizen's Second Amendment rights if they pop up on this list.
Unless the very existence of the nation is at stake, and it certainly isn't with the people on the watch-list, NOTHING should ever rise above the protections of the Constitution.
The Government has never made a case for why they MUST have this watch-list, and deny the estimated thousands of people on it their Constitutional rights.
But, it exists anyway.
The real danger in this watch-list is that it might become precedent, and other secret lists of banned persons may (or may have ALREADY) be developed, lists which ALSO evade Constitutional protections.
It was not smart to trust George Bush's Government with such power, and it's even less smart to trust the current gangsters in DeeCee with a free get-out-of-Constitution card.
The list must be deleted, and never maintained again. Only when it, and any clones (it is hard to imagine that there are no others) are gone, will American citizens again be able to say that they have a Constitution that fully protects them.