Nate Silver's reading:
Mostly I simply think that the disaster is reinforcing people's frustration -- an emotion that has become very widespread within the country, and which crosses most demographic and political boundaries. If that remains the prevailing mood of the country in November, the risks to the incumbent President and his incumbent party are mostly to the downside.
Covering the same ground, Steve Lombardo lays out reasons why the spill isn't comparable to Katrina:
Team Obama has something that Bush never had with Katrina: a villain. A large international oil company. What better villain could you ask for? Team Obama will maximize this to its political advantage over the coming weeks. With Katrina, Bush was the villain.