Letterman Backstage     Why did hundreds of fans of CBS' "Late Show With David Letterman" refuse to budge from barricades outside the Manhattan studio's stage door after the early taping of the Friday June 11 show?     Cynics may say they were waiting for a glimpse of Sean (P. Diddy) Combs or Paul Anka but more likely they were pining for the show's third prominent guest who had just made his 10th appearance.     Sure enough, cheers went up -- along with a few mutters of "Who's that?" -- when Jim Gaffigan and his wife, Jeanne emerged, to follow their 5-week-old daughter into the waiting limo.     Earlier backstage during the show, Jim was busy recruiting future nannies for his young daughter. You know how hard it is these days to find role models.     Also contemplating Letterman's youngest ever backstage guest was fellow comic (and Letterman talent coordinator) Eddie Brill, who has his own website.