It is a slippery slope. </echo> But in this case, that slope is on a hillside in a Middle East country, not American soil. If al-Awlaki wants to avoid a bomb up his bum courtesy of a Predator drone, he can sneak his ass back into the country and turn himself in. As long as you are A) in a foreign country and B) waging war upon the territory and peoples of the United States of America, you're fair game, as far as I'm concerned, citizenship be damned.
There's an outstanding question of when does the government have the authority to rescind an American Citizen's citizenship. The answer is, when the citizenship was obtained through naturalization and obtained fraudulently. Other than that, there's no legal mechanism for stripping an American Citizen of his citizenship.
Al-Awlaki is a natural born American. Hell, there's more documentation proving him eligible to be the President than there is on Obama himself. For Al-Awlaki to cease to be a legal American Citizen, he would have to explicitly... file paperwork to that effect. Hey, it's a bureaucracy. SCOTUS has opined several times that the only way for a legitimate American citizen to be divested of their citizenship is for them to do it in an unmistakable and unequivocal way, in triplicate. Taking foreign citizenship used to do it, not anymore. Being convicted of treason used to do it, not anymore.
Think of it this way, if the government (god forbid) were to declare war on Right-Wing Americans and start the confiscations and roundups FEMA camps and what not, if a band of American patriots were to organize and fight back against the federal troops (who obeyed the unconstitutional orders), I think we would all agree that they would be declared terrorists, and rightfully so. (cue V for Vendetta quote here.)
Would those American patriots fighting back against the American government on American soil automaticly lose their citizenship just because they have set themselves against the sitting American government? Should they?
When they "Nudge. Shove. Shoot.",
Don't retreat. Just reload.