My guess, without seeing "The Soup", is that maybe it's in reference to "Farrah's Story", and that Kate Jackson was in Charlie's Angels with Farrah, and also a breast cancer survivor?
am I close?
I'm curious to know why letterman would bring up Donna McKechnie. But this is the guy who spouts things like, "And on the back of the CD, you can see Linda Ronstadt's house! I'm in love with my cowboy hat!" No lie, that's a direct quote.