In 2003 it was announced that Robert Stack would be appearing on the Simpsons:
http://www.people.com/people/article/0,,626449,00.html http://www.ew.com/ew/article/0,,465629,00.html http://animatedtv.about.com/cs/guests/a/guests03.htm
He was already dead when these press releases were announced, so they obviously had recorded his voice already.
WTF happened? I was looking forward to it. Don't let his last role be in Butt-Ugly Martians.