From Muppet Wiki

David Letterguy is the host of "The Alphabet Roadshow," a reality series that features ordinary people reciting the alphabet in unusual ways. The crew, which consists of Fran and Frankie, travels from location to location in search of those who exhibit the talent to warrant a feature spot on the program. Letterguy made his way to Sesame Street in Episode 4125.

David Letterguy's name is a reference to David Letterman.