To sum it up:
New Orleans is mostly below sea level, protected by levees which keep the water out, and pumps which remove the water brought in by rains.
Katrina, a category 5 hurricane (the biggest they get, and judging by the high water temperatures in the area, it’s not going to get any smaller), is headed towards New Orleans. Like all hurricanes, it will bring massive rains, massive waves, and will most likely drown the city. The levees protect against tide, waves and storms, but 20+ foot surges are probably too much for the city.
In 1969, hurricane Camille was the last and one of the few cat 5 hurricanes to hit the mainland. It mostly spent its time beating up semi-rural areas, and did not, IIRC, nail New Orleans.
http://www.geocities.com/hurricanene/hurricanecamille.htm
In essence: it’s very likely that New Orleans will simply fill with seawater, the water destroying what the 170+mph winds haven’t. Recoverable? Probably, but this is going to be fucking expensive. Pray for the people, and pray for the insurance companies.
There’s also the original discussion in P&R, but I don’t trust them to stay focused on the topic. Most likely it will end up a global warming/blame Bush thread.