(1) What does "preventive detention" allow?  

It's important to be clear about what "preventive detention" authorizes.  It does not merely allow the U.S. Government to imprison people alleged to have committed Terrorist acts yet who are unable to be convicted in a civilian court proceeding.  That class is merely a subset, perhaps a small subset, of who the Government can detain.  Far more significant, "preventive detention" allows indefinite imprisonment not based on proven crimes or past violations of law, but of those deemed generally "dangerous" by the Government for various reasons (such as, as Obama put it yesterday, they "expressed their allegiance to Osama bin Laden" or "otherwise made it clear that they want to kill Americans").  That's what "preventive" means:  imprisoning people because the Government claims they are likely to engage in violent acts in the future because they are alleged to be "combatants."  

Once known, the details of the proposal could -- and likely will -- make this even more extreme by extending the "preventive detention" power beyond a handful of Guantanamo detainees to anyone, anywhere in the world, alleged to be a "combatant."  After all, once you accept the rationale on which this proposal is based -- namely, that the U.S. Government must, in order to keep us safe, preventively detain "dangerous" people even when they can't prove they violated any laws -- there's no coherent reason whatsoever to limit that power to people already at Guantanamo, as opposed to indefinitely imprisoning with no trials all allegedly "dangerous" combatants, whether located in Pakistan, Thailand, Indonesia, Western countries and even the U.S.