BlueGTP91 and Lawngoose you obviously have no daughters, I have 3 daughters and find no humor in statutory rape and impugning the morals of minor children. The "joke" was not intended to be funny it was a verbal bomb intended to demean Palin. The laughter you heard was that of an embarrassed audience. As for the apology, its simply not believable.
If you can see Letterman laughing at jokes about his own son's vulnerabilities or making jokes about Obama's daughters, then you go ahead and believe his apology was sincere. If not, just admit there's a double standard when the media elite makes fun of a conservative.