if racists were rational in their beliefs and could be convinced they were wrong with logic. The problem is that racism is by its very nature an irrational belief.
As far as the "it's a joke" defense some are giving for it, I am willing to excuse a little bigotry for the purpose of comedy. It's important not to take ourselves so seriously. The problem with this defense is that the "joke" isn't funny at all, not in an "I'm offended" way, but in a just plain old not fucking funny way. Plus, with the defense the cartoonist gave that it wasn't talking about Obama (his words were something to the effect that if it were about anyone it would be Pelosi, which it isn't) what's the joke? There's no there there.
Pretending that cartoon wasn't racist is an insult to the intelligence of clearly thinking people.