To the last post, you’re not quite right about the Geneva Conventions. First, yes, there are four defined criteria for attaining combatant status. However, they’re not hard criteria: you don’t need all of them to be considered a combatant. At a minimum, open display of arms is needed, but the others have been more flexible in the past.
Also, you missed Falk’s point about occupation. While whether there is a civil war in Iraq is debatable (I don’t think there really is, but it’s a fluid situation anyway), it nevertheless is an occupation. And resistance to occupation is most certainly protected under the fourth Geneva Convention as well as the Hague Convention of 1908 (I think it was that year).
Be careful about sweeping generalizations about the insurgency. Zarqawi is the head of a group, not the entire resistance. This is an organization much more akin to network warfare than traditional command structures.
Finally, you’re only analyzing one part of the Geneva Conventions. You make the mistake of creating a third category: they’re not combatants, and they’re not non-combatants (civilians), so they are terrorists. But, there is no terrorist category in international law, barring possibly Annan’s latest statement in “In Greater Freedom,” but that has not passed into customary law. Rather, there is an acknowledgement of “terror tactics,” but where this comes into play is in military discrimination. International law only has two categories - combatants and civilians, and if you’re not in one, you’re in the other. The Convention does include protections for civilians, and that includes a fair trial, although recognizing the particular problems of military occupation for carrying out trials.
That said, once you define what category these individuals fall under, you can either prosecute them for war crimes (if combatants) or domestic crime (if civilians). But you cannot create a legal black hole for “terrorists” where nothing applies. Remember, you do that, and you destroy the rule of law, which is ostensibly something that the US troops are fighting for in Iraq.