1) That's not true.  There have been a lot of extensions of constitutional power that apply to illegal immigrants, foreign people in the country, prisoners of war, and whatever else you can imagine, although that is an issue that the courts have to address.  They did address it as recently as saying that the Guantonomo detainees were entitled to legal counsel. 

Your treason point is valid but the definition of treason is something along the lines of a citizen intentionally or knowingly betraying the U.S.  You can't exactly betray someone else's country...being a citizen of the U.S. is a prerequisite for the crime.  Treason isn't truly defined in the Constitution in terms of a crime in the procedural sense.  There are limits placed on what the government can do procedurally in terms of prosecuting treason and how it can define treason, but that isn't what you would call a Penal Code definition of treason.  There is no punishment for treason specified in the Constitution (or any kind of sentencing guidelines), so a prosecutor wouldn't even be able to use it.  There are federal statutes that do that.

2) Article 3 Section 2 has very little to do with what the judicial branch actually ends up doing.  The Constitution drafters essentially drafted it as vague as they possibly could and the courts just kind of went with it.  Judicial case law and Supreme Court decisions interpreting the Constitution are a much better guideline of what kind of authority the courts have rather than the Constitution itself.  The Supreme Court has the ability to draft its own rules of procedure since Congress gave them that power (which the Supreme Court then outsources to a larger judicial body).

3) You are excluding a lot of examples, and there have been a lot of rulings on cruel and unusual punishment.  Electrocuting people in an electric chair has been interpreted as cruel and unusual by some of the states, as has hanging people.  The Supreme Court hasn't stepped in to stop them.  If anything the definition is much broader today than it was then, since we react even more sensitively to cruel and unusual punishment than people did hundreds of years ago.  Its an evolving standard (like just about everything else in the Constitution), and whether or not waterboarding is torture is a constitutional issue.  Not to mention it isn't listed in the Army Field Manual as an approved interrogation method.

1) Extensions of constitutional power are typically fairly explicit in purpose and scope though, and I don't remember ever hearing one that extended the penal system to non-citizens outside of the scope of immigration which does so intrinsically.

Either way I kind of think this argument in general is begging the question.  Without an agreement on the classification of waterboarding the legality of torture or cruel and unusual punishment is moot, which is why skipping to that begs the question of whether it matters in the context of the topic, which I would disagree that it does..

2) Not really what I was saying, but no worries, I think my point here was moot.

3) I actually took those examples directly from the Wiki page, another more comprehensive list included: breaking wheel, boiling to death, flaying, disembowelment, crucifixion, impalement, crushing, stoning, execution by burning, dismemberment, sawing, decapitation, scaphism, or necklacing.  I took the time to review each of these, distateful as it was, and am more convinced of my position than ever for it.

Pretty clear we've discovered some gruesome things to do to each other over the centuries, and I honestly don't think waterboarding belongs on the list or really anywhere near it.  Prolonged, severe, and excruciating pain ending with death is the underlying theme here, and while certainly I would agree it doesn't have to have all of those, I would expect something classified as torture to qualify for at least one of four among excruciating pain, maiming, causes death, and prolonged exposure to the act.

If panic attacks and depression are the worst side effects (which from what I've read are about the extent of it) then I'd say you've found yourself a mild hazing ritual.  I would say that electrocution is substantially more torturous than waterboarding and yet from the numbers I can find we've had nearly 170 executed that way since 1979 With the last electrocution (that I can find) having occurred about 6 months ago in South Carolina.  I would say that if electrocution fails to meet the constitutional standards of torture and cruel and unusual punishment then there is not even the basis to argue for waterboarding meeting such a standard.  That's not to quash a moral or principled argument, but I do find a legal argument to be fairly hollow given those facts.

Usually I feel as if people oversimplify arguments. In this case, you are trying to overcomplicate something to justify horrendous behavior.

Indeed, not to mention the majority of the examples sqrl listed for cruel and unusual punishment were outlawed before the 20th century.  Cruel and unusual punishment has been an evolving standard and has a much broader definition than it did even at the beginning of this century.  Wikipedia's extent of getting into the cruel and unusual punishment issue is cursory at best and doesn't provide the breadth or rigor of analysis that the court system has used on the issue.  Using it as a valid source on this issue is questionable at best.

And death penalty punishments are a completely separate issue than torture methods.  Courts do not treat them the same, so saying that courts allow one isn't a valid defense to say that they should allow the other.

Its not begging the question whether or not these things can be extended to non-citizens.  Do you think the American people would stand by if waterboarding was used on domestic citizens in police stations as part of interrogation proceedings?  Even the military is not allowed to use waterboarding according to the Army Field Manual.  Its completely relevant.

Well thank god we outsource interrogation then.

http://www.washingtonpost.com/wp-dyn/articles/A41215-2004Aug4.html