I'm sorry, but the entire punditocracy has missed the point.
It is NOT news that someone in show business makes an intensely rude, crude, lewd, insulting, disgusting and/or despicable comment regarding an entirely inappropriate subject. Indeed, if we demanded an apology every time that happened, we'd never get anything else done.
The point is that Letterman's comment is a shot across the bow. He is saying, "Governor Palin, if you so much as poke your nose outside the great State of Alaska, we will insult and smear and humiliate and hound you and your family and your community and anybody who thinks or even looks like you until you feel so degraded and miserable that you run back home with your tail between your legs and never, ever come back. We hate you, we fear you, we loathe you, and we will stop at nothing to destroy you."
That's the news.