I read this excellent article from Bill Simmons the other day, it's dated April 8:
http://grantland.com/features/david-letterman-retires-cbs/
Now we’re already thinking about the Next Next Guy — within 12 hours of Letterman’s stunning announcement, names like Tina Fey, Neil Patrick Harris, Ellen DeGeneres and even Conan were being thrown around for his job. I will spare you the suspense — it’s going to be Colbert. Well, unless he has three more Twitter debacles or something. He’s getting that job.
http://grantland.com/features/david-letterman-retires-cbs/