Few lessons seem to have been learned from past disasters, including the inescapable conclusion that unthinkable outcomes do happen despite their being highly unlikely. Apparently the planning for New Orleans only took into account a Category 3 hurricane, not a Cat 4 or Cat 5.
Regardless of plans that may have been made, experienced disaster relief organizations were unable to provide services in New Orleans. The article by Ann Rogers of the Pittsburg Post-Gazette describes a little of what has happened. Snippets: