One possible explanation for the surprise decision at the Guantanamo Bay tribunals today to throw out the case against 20 year old Omar Khadr is that the legislation passed last year to set up the tribunals was trying to be too clever by half, in seeking to make the Gitmo setup slightly constitutional but covering the possibility that Gitmo could be shut entirely in the future, meaning that the same system would be needed to cover terrorism suspects held in the United States.
Hence the change in definition of who can be tried from "enemy combatant" to an "alien unlawful enemy combatant" -- a definition apparently designed to ensure that it would allow non US citizens (including Green Cards) to be held under the military detention system on US territory (why else specify aliens?). But, in an oversight that bears all the hallmarks of Bush administration incompetence, the military judge pointed out that the government had forgotten to reclassify the detainees from the old definition to the new one: case dismissed.
If the Bushies were smart, they would spin this positively as neat disposition of an embarrassing child soldier case (as Khadr was when he committed his alleged offences) and an example of tribunal independence. More likely, they will stomp their feet and look to find a scapegoat for the problem.
UPDATE: Legal analysts suggest that the key missing word for existing detainees is "unlawful" rather than "alien." Which prompts the question of how a tribunal that is by construction prior to the military commission is going to make a judgment as to whether a combatant broke the law, and if so, what law? Also, as pointed out by by one of the defence counsels, one problem here is that the legislation governing the tribunals was rushed -- part of a pre-election stunt by George Bush in the runup to the 2006 mid-terms.