While refuting Glenn Greenwald, Andrew Sullivan frames the administration case against al-Awlaki in a way that makes a lot of sense:
The CIA and the military have not been authorized to kill any US citizens on American soil. But it is utterly uncontroversial that the military can kill a US citizen abroad if he is waging a treasonous war against the United States (see: Ex parte Quirin [1942]). Obama, moreover, has specifically rejected the dictatorial "enemy combatant" powers asserted by Bush and Bush alone, and expanded judicial review of this kind of military action, hence the lawsuit currently being filed by Awlaki's father.
Food for thought. I still don't like the smell of this case, but "waging a treasonous war against the United States" is a pretty strong argument in favor of the administration's position.