As I understand, Letterman tried to cover his distasteful mockery by claiming the "joke" was directed at Bristol who was over 18 at the time and not the younger child, Willow. Of course, Governor Palin saw it otherwise.
Either way, Letterman was and is lower than pond scum. The bulk of my points in the post still stand when it comes to the relentless barrage of attacks from the left on all of the Palins.