One of the major problems with Hurricanes is not so much the high wind but the storm surge. With all the redundancy in typical structures they can survive wind loads above their orginal design loads. I don't know of too many buildings which have been design to resist a 25' high wall of water. Note I don't know how high of surge that Katrina produced but I seriously doubt my house could survive what ever the actual storm surge was.
RARSWC, you bring up a valid point. However, unless the home is directly next to a broken levee/seawall, the water rises fairly uniformly around the house, meaning there isn't a net force on the house. A problem would be if your home was watertight, and the water on the outside rose while the inside was dry. So basically, before you leave, I'd recommend opening the windows and doors.
