Was the killing of Anwar al-Awlaki an unconstitutional violation of a citizen's due process rights?

By all accounts Anwar al-Awlaki was a bad guy - at least for those on our side - but was his (apparent) killing illegal? Is the United States now free to assassinate its own citizens if a presidential administration deems it acceptable? Critics argue that this action violated the Fifth Amendment's Due Process clause. Even more so, some wonder if an American citizen been killed for the content of his speech.

Former Bush legal adviser John Yoo thinks the killing was justified. Here's a summary of an editorial:

A U.S. drone strike Friday killed Anwar al-Awlaki, a Yemeni-American linked to the shootings at Fort Hood, Texas and the attempted bombing of a Northwest flight. It was a victory, writes John Yoo in The Wall Street Journal, yet "even as details of the operation leaked out, critics claimed that our government had 'assassinated' an American citizen without due process."

The ACLU has represented Awlaki's father, arguing "the Constitution forbids the government from trying to kill an American citizen for allegedly joining the enemy." The U.N. has suggested drone strikes deny due process. Last December, a court threw out the ACLU claim, saying "Awlaki always had the option of returning home to prove his innocence." American officials have defined an assassination as "an act of murder for political purposes," something which Awlaki's killing was not. "American citizens who join the enemy do not enjoy a roving legal force-field that immunizes them from military reprisal."

Abraham Lincoln knew early on that the government had the right to "treat its own citizens as enemies when they take up arms in rebellion. Supreme Court opinions have upheld Lincoln's principle." The shape of the war on terror, with a stateless enemy that recruits fighters from any country makes Lincoln's idea more important, and in 2001, the Supreme Court reaffirmed the idea that someone could be at once a citizen and an enemy combatant.

The Obama administration should be certain a citizen is a member of al Qaeda, and it should emphasize capturing not killing enemies, but as they face a drawdown, "the U.S. may be left with no opportunities for capture, and precious few chances to kill."