Answer: Anyone. The Washington Post is reporting that over the weekend the White House and its House allies successfully watered down an earlier version of the military commissions bill to include a less restrictive description of how the government could designate civilians as "unlawful enemy combatants."

The [unlawful enemy combatant] definition applies to foreigners living inside or outside the United States and does not rule out the possibility of designating a U.S. citizen as an unlawful combatant.

That means that anyone the government decides "has engaged in hostilities or who has purposefully and materially supported hostilities against the United States" [ital mine] or its military allies can be detained indefinitely. In other words, a U.S. citizen in the United States. And they've successfully defined the "battlefield" as anywhere and everywhere. Plus, if Congress writes and passes it, it's unlikely the Supreme Court would step in and declare the definition unconstitutional. I'm less enthused by this military commissions compromise by the minute.