He admitted such, in his book, I cannot remember the name, but it is common knowledge. That was the original accusation. Most of the blogs have something in it in their archives, I'll leave it to you to do the research.
With an issue like stem cell research, tempers will fly high, and people will not be in their right minds talking about it. Rush did not make fun of him, he imitated him, and accused him of dramatizing the effects of his disease.
But back to the O/P, Letterman has the right to say what he did, and Palin has a right to call him pathetic. Seems like in the long run, this episode will be better for her than it is for him. I encourage him to continue.