I'm extremely skeptical. Santorum is an idiot, and I don't think he's exactly Mr. Equality Sunshine, but I have a very, very hard time believing that the slip was due to nearly calling the President a racial slur by accident. The GOP candidates have been campaigning for months, they have been going non-stop, and as we get closer to the convention it will only get worse. I've been known to stumble over my words when I'm tired, and I know it's not uncommon.
Don't get me wrong, I can't stand Santorum, but I'd rather loathe him for the policies I know he supports, rather than speculation on a word he didn't even say.