"We are always looking for someone without a movie to plug," said "Late Show" executive producer Rob Burnett.
It appears to be Rumsfeld's first foray into late night TV — where comedy is king and heavyweight talk is usually avoided.
As the architect of the Iraq war, Rumsfeld will likely talk with Letterman about the insurgency there and stump for the administration's stay-the-course policy.
"I'd be surprised if it was all light-hearted," Burnett said last night. "And I'd be surprised if it was all serious."
A Pentagon spokesman Wednesday declined to comment on Rumsfeld's appearance until it appears on his official schedule, probably late Friday.
Burnett said the show has been working on getting Rumsfeld "for several years."
Why did he say "yes" now?
"It was really a question of scheduling," says Burnett. "There is no particular agenda."