What Is It With David Letterman And Sarah Palin?

I've accidentally watch David Letterman the other day and he was telling a joke about Sarah Palin. I've also read in the LV Sun and RJ, when they have comments from the late night show and invariably, there is Letterman making fun of Sarah Palin. Why?
I guess, given the sexual harrassment at by Letterman at work, I kind of think he has a problem with women in general and strong conservative women in particular.
Letterman is just a rich pathetic human being, who has some serious problems with women. And BTW is really not that funny.