In regard to the delayed levee breach with Katrina, CNN was explaining that this morning.

With Katrina, the eye passed just to the east of NO, which made the winds primarily southerly and a large rain event - all of the rain and the wind caused the water to flow south from Lake Pochatrain (sp?) and force it down into the city from that direction - the rain therefore took longer to fill the lake and flow south.

With Gustav, the eye passed just to the west, creating a surge into NO from the ocean, which is a more immediate effect. The rain itself will not be as much of a worry as the initial surge itself.

This does not mean they are in the clear by any means - the loose barge and any number of things could easily cause problems with the levee system, but the biggest concern is in these earlier hours.

Hope that helps!