<< He is a POW. By federal law, someone is expatriated when they are members of an armed force engaged in hostilities against the US. John Walker is no longer a citizen, and has to be treated like any other POW. >>
I agree. He renounced his citizenship the moment he took up arms with the Taliban. One has the right to renouce ones citizenship and fight with a foreign power if one so desires, and so long as it is not done during the course of hostilities such an act is not treasonous (although there are other consequences involved in taking such an action of course). Since his renouncement of citizenship and joining of the Taliban took place before 9/11, his subsequent actions cannot be termed treasonous.
So what is an appropriate punishment for young Mr. Walker, since we can't honestly call him a traitor? Well, he's already forfeited his citizenship, i propose going one step further, he be officially declared by act of Congress Persona non grata in the U.S., its territories and holdings, transportation carriers (i.e. passenger airlines), and/or any U.S. government sponsored/maintained sites or functions, anywhere in the world. If he so much as steps foot on U.S. soil, he should be subject to being shot on sight.
Since he's declared himself a Taliban and renounced his American citizenship, let him suffer the same fate as those whom he chose to fight alongside. If we wind up turning over all the Taliban POWs we capture to the Northern Alliance, then guess where Johnnie Walker is going. If the Taliban POWs go before War Criminal Tribunals, then guess who goes along with them. He'll probably wish that we did bring him back here to face trial as a traitior, and a potential quick death by a humane execution once he finds out what the alternatives are.
As the judges say, "May God have mercy on his soul."