This seems to fall into the category of "snatching defeat from the jaws of victory." If the goal of John McCain's campaign is to separate himself from the current Administration -- and to cast himself as an agent of "change" -- then the hurricane gave them an unexpected "out". Prior to the hurricane, it would have been hard to imagine a scenario where McCain could have avoided a Bush speech -- but that opportunity fell into his lap. And now, Bush is going out of his way to deliver a video speech?
This strikes me as poor strategy. Regardless of how you and I feel about him, the bottom line is that the president is (fairly or not) extremely unpopular right now.
There is no doubt that any historical look at President Bush will include Hurricane Katrina. Could it be that he is hoping to use the improved hurricane response as a way to begin restoring his legacy?