There's a lot of material available online about the Geneva Conventions, including their full text. One site seemed particularly well-organized and useful: http://www.genevaconventions.org/.

One thing that it's interesting is that I couldn't find any definition of "illegal combatant". I suspect the Bush Administration made up that term to advance the argument that it can define for itself who is and who isn't subject to the Conventions. I really doubt that--the Conventions have a definition for just about anybody connected in some way with a conflict, and then closes off any wiggle room with this clause:

Article 5

The present Convention shall apply to the persons referred to in Article 4 from the time they fall into the power of the enemy and until their final release and repatriation.

Should any doubt arise as to whether persons, having committed a belligerent act and having fallen into the hands of the enemy, belong to any of the categories enumerated in Article 4, such persons shall enjoy the protection of the present Convention until such time as their status has been determined by a competent tribunal.

And I doubt that Bush can simply declare himself to be the "competent tribunal". I would interpret Article 5 as saying that anybody who can't be classified as a combatant under the Geneva Conventions must either be treated as a civilian or be turned over to an international tribunal such as the International Criminal Court for prosecution.

I don't see a loophole.