"Here's a little known fact - Arnold is the first body builder to run for governor since Janet Reno"

In this quote, David Letterman is making an amusing observation about Arnold Schwarzenegger, who was a famous bodybuilder before becoming the guv of California. Letterman mentions that Schwarzenegger is not the first bodybuilder to run for governor, as Janet Reno, who was likewise a bodybuilder, ran for guv of Florida in the past. This reality might not be well-known to many people, hence the phrase "unknown truth." Letterman's comment is indicated to be lighthearted and poke fun at the concept of a bodybuilder becoming a political figure. In general, the quote showcases Letterman's wit and humor.