It has always been a dicey proposition for talent contractually bound to one network to moonlight on another. Ordinarily a cable network will routinely allow its stars to take regular jobs on a broadcast network for the larger exposure that such appearances receive. CNN's Anderson Cooper, for example, is a regular contributor to CBS''s 60 Minutes; NPR/PBS analyst Cokie Roberts is also on the payroll of NBC News; PBS talk-show host Charlie Rose is co-host of CBS This Morning. But as cable TV continues to grow its audience, it is likely to think twice about allowing its stars to compete on broadcast outlets. Today's (Friday) New York Post reports that CNN chief Jeff Zucker has banned all CNN and HLN personalities from appearing on ABC's Good Morning America, thereby blocking Nancy Grace from her popular legal debates with Dan Abrams. And, in a similar exclusivity deal signed last month following his appointment as coanchor of ABC's Nightline, Abrams, too, agreed to shed his interest in Abrams Media, an assortment of websites including the popular Mediaite.com that he founded four years ago.