So the Morganza Spillway is being opened, probably as I type. Much of the Mississippi is going to rush down the Atchafalaya River and the giant floodway around it. The delta will do as deltas do during floods - it is going to be flooded. It was not easy to make this decision, which will flood Morgan City as well as several other towns and many acres of agricultural lands.
Not an easy decision... but considering the alternatives, the only one that could have reasonably been made. By not opening the spillway, the river might have put New Orleans under more water than Katrina, and inundated Baton Rouge as well.
People in the area can describe the issues much more accurately than I, so see here for more info.
Now it's time to wait, and watch, and hope the Old River Control Structure doesn't fail. If it does... we've got a whole lot of trouble coming.