In 2006, I posted an item about the future PBS series called Make 'Em Laugh: The Funny Business of America. Current updates reveal the series appears to be ready for the upcoming 2008-2009 PBS season. This is a November 2007 article about the series.
Also, Current.org has the program listed for showing in the winter/spring PBS season of 2009. The six hour, six-part series is to cover comedy from the early stage to current films, but not in that order. It appears to be a look at America comedy rather than comedy, in general.
"Make ’Em Laugh doesn’t move chronologically from vaudeville to Jon Stewart but divides its subject by genre, with episodes on physical comedy, satire and parody and sitcoms, among other topics. “I was afraid if we broke it down by decade, it would get predictable,” Kantor says. “With comedy, we wanted to have more surprise.” - Producer, Michael Kantor, Pipeline (current.org)
Update: August 6, 2008 - From the TV SQUAD website, Billy Crystal is listed as the new host for Make 'Em Laugh: The Funny Business of America. The six-hour documentary is said to air in January 2009.
October 2008 - Update on the Make 'Em Laugh series and new book.