A number of pundits, such as our own Nate Silver and Slate's John Dickerson, have argued that Gustav could end up benefiting the GOP politically. But now that the situation seems to be stabilizing, I'd argue that it's shaking out about as well as it could have for Obama (with the possible exception of another legacy-destroying fiasco for Bush, which no one was rooting for).
Had the damage been more serious, McCain would have gotten to play president for the week--there was talk of him delivering his acceptance speech from New Orleans, and he would surely have spent significant time in the affected region. (All the Gulf Coast governors are Republicans, which would have made travel relatively easy for him and difficult for Obama.)
Had there been no Gustav, on the other hand, McCain would have had to deal with Bush and Cheney tonight, but the rest of the week would have been an extended Obama-bashing session.
As it happens, it looks like McCain is basically going to have to put on an orthodox convention--no heroic imagery from the Gulf--but with one hand tied behind its back, since it would be poor form to lay into Obama a day after declaring politics off the table.
Obviously you never want to see a tragedy of any kind, even if it turns out to be manageable. But, from a crass political perspective, Obama is probably better off today than he was over the weekend.