The aftermath of Hurricane Katrina saw both National Guardsmen and regular Army personnel being used to provide police services in the disaster area. News reports included incidents where members of the US Army 82nd Airborne badgered and threatened members of the media covering the recovery of bodies in the flooded city of New Orleans (see the news story from San Francisco Chronicle, "As Bodies Recovered, Reporters Are Told, 'No Photos, No Stories'.")
While the presence of regular military personnel undoubtedly lead to the restoration of order in the post-hurricane chaos, it still is uncertain whether the use of such troops is legal. The Posse Comitatus Act of 1878 prohibits the use of the regular armed forces to execute or enforce civilian law unless "expressly" authorized by the Constitution or Congress. In 2003 Stephen Young (reference librarian at The Catholic University of America, Kathryn J. DuFour Law Library) presented a fairly exhaustive overview of the law in post-9/11 America. Individuals interested in the Posse Comitatus Act should check out his article, The Posse Comitatus Act: A Resource Guide, at the excellent web site LLRX.com.