The only time I remember him getting truly annoyed with a guest was back in 1993, when Rush Limbaugh -- who had earlier compared Chelsea Clinton, a then-gawky 13-year-old with braces, to the hood ornament of a car -- made some terribly cruel comments about Hillary Clinton's weight, to the obvious consternation of the studio audience, which began to boo him.

Letterman quickly seized control of the situation by silencing Limbaugh with his own cutting remark, "You know, you can say that because you are the finest looking human specimen on the planet." The crowd cheered, and Limbaugh likely learned a big lesson that night, which was that his was a face made for radio.

I'm looking forward to seeing Stephen Colbert as host of "The Late Show," once he assumes the role in September. But I also hope that Letterman won't just disappear after tonight's final sign-off, the way Johnny Carson did after his last show in 1992, and that he will make an occasional appearance in his old haunts for old times' sake, just to let his many fans know how he's doing.