I've always liked Letterman and have preferred his show to both Leno and Carson over the years. His joke about A-Rod knocking up the Governor's daughter was just another joke in a long line against the Palin family. One of the jokes was going to go too far...unfortunately it was Letterman's.
During the election, Obama simply stated to the press that his family was off-limits and the press honored it. Hell, they honored everything he did to the point of worship. Palin's family was the subject of a great media focus...most of the time unfairly.
Letterman apologizes and now finally Sarah Palin's family is off-limits because one joke could be taken to mean something that was beyond the taste of the average American.
I hope the Palin's enjoy their time of being left alone and I hope this joke doesn't become a Don Imus moment for Letterman.