Anyway, enough time for a postmortem if/when the Mississippi dies (long live the Atchafalaya)! I've seen several consistent versions of the story; as usual I am willing to take Jeff Masters as definitive.
America's Achilles' heel: the Mississippi River's Old River Control Structure
There's apparently a whole lot of industrial infrastructure between Baton Rouge and the Gulf that relies on a whole lot of water coming by. If the Big River reroutes itself above Baton Rouge, it will totally destroy the communities it rolls over and damage a lot of petroleum infrastructure. Masters points out that it will have a huge impact on the areas that lose access to the flow as well.
Louisiana was not a wealthy place before Katrina, not to mention not historically well-governed, and of course the present economic turmoil helps nobody. It's a bit mind-boggling to imagine the worst case scenario for the next few months. I hope it doesn't happen, but it might.
Update: David Brin, coincidentally the author of the quote of the week above, says to let the river go. Good luck with that one.