If we reviewed transcripts of past Letterman shows, would we really find no jokes about Chelsea Clinton, the Bush Daughters, or even other jokes about the Palin's that went under the radar?
The real solution for me is for Mr. Letterman to start having teenage guests, even better of the non-celebrity type, teenagers who could be considered role models for other teens, and even adults. Or at the very least, a solid week of at least one teenage girl role model appearing on Letterman's show.
It seems the longer that our "hallowed" TV personalities such as Letterman and Oprah Winfrey are on the air, the more hollow they become as they avoid interacting with anything that is really new or different from what they have already done.
Unless Letterman actually periodically has female teenage guests on his show, and perhaps as on location field reporters where they are not the joke, nothing has really changed in my book.