He was not a combatant though, and he wasn't even in any of the countries we were "at war" with. So if the government can use the pretext of "bad person sides against US Government in Youtube videos" to reach out and assassinate the person anywhere on earth, (because global war on terror = whole globe), then they have de facto denied Constitutional rights to any US citizens who the government chooses to go after, and does not require the US Government to prove anything.
Also, as has already been said in other posts, the Al-Awlaki family begged the US Government for over a year to charge him with a crime, ANY CRIME, so that he could turn himself in and have his day in court. The US Government refused to ever charge him with a crime specifically so they could deny him his rights.
You guys need to stop pretending this was some American killed on a battlefield while shooting at US soldiers. All it does is make you look dishonest, so what is the point?