Optimus Prime on Letterman?

I was watching the Boston 4th of July celebration and there was a commercial for the Late Show with David Letterman saying that Optimus Prime will be reading off the Top 10 list Monday night. I am not sure if they mean some CGI thing with Prime (and probably using the first or second movies scenes) or if Peter Cullen is stopping by and reading the list in Prime's voice. I guess we will have to watch Monday night find out.