As an Obama fan, I am offended by this cartoon. And I understand why his campaign would be, too. However as an artist myself, dabbling in cartoons, I think this cartoon is successful. It works because it contains an element of truth — what many (ignorant) people think about Obama. That's what political cartoonists do, they use cartoons to ridicule. This one is brilliant because it's touched a nerve.
Now the New Yorker might follow-up this cover with a cartoon portraying the average American who really believes the things pictured in this cover. Now that would be funny, too, particularly since the ridicule was intended for them, and not Obama.