Considering the latest public debate I did a little research. The following is from the State Department website. It raises the question of whether Al-Awlaki WAS a U.S. citizen at the time of his death. The law probably needs to be updated to explicitly cover service in non-state organizations engaged in hostilities against the U.S. However, it's possible that his joining a belligerent Al-Qaeda and taking up arms against the U.S. was evidence of an intention to relinquish his citizenship.
"Although a person's enlistment in the armed forces of a foreign country may not constitute a violation of U.S. law, it could subject him or her to Section 349(a)(3) of the Immigration and Nationality Act [8 U.S.C. 1481(a)(3)] which provides for loss of U.S. nationality if an American voluntarily and with the intention of relinquishing U.S. citizenship enters or serves in foreign armed forces engaged in hostilities against the United States or serves in the armed forces of any foreign country as a commissioned or non-commissioned officer."
"Military service in foreign countries, however, usually does not cause loss of citizenship since an intention to relinquish citizenship normally is lacking. In adjudicating loss of nationality cases, the Department has established an administrative presumption that a person serving in the armed forces of a foreign state not engaged in hostilities against the United States does not have the intention to relinquish citizenship. On the other hand, voluntary service in the armed forces of a state engaged in hostilities against the United States could be viewed as indicative of an intention to relinquish U.S. citizenship."