Hey, Todd Palin...
don't you think Letterman's joke about one of your daughters getting knocked up was probably about Bristol, one of your daughters who was actually knocked up and is now an unwed, teenage mother?
And to go from a joke that talks about being knocked up to using the word 'rape' is a little extreme, don't you think? You do realize that conception can happen during consensual sex too, don't you?
In short, take it easy, grandpa.