Anybody know the law behind the Anwar al-Awlaki thing?

Does anybody know the story behind the "secret memo?" This is one of those subjects I just can't discuss with my non-lawyer friends. They all seem to think there is this secret set of laws out there that nobody knows about.

To the best of my knowledge, there aren't any "secret" laws. It's pretty much all out there on Westlaw if you know where to look.

Anybody have any idea what the legal theory behind this is?