Janet Jackson's current CD ''Damita Jo'' may have suffered lackluster sales, but she could always return to her old job, sitcom actress. In fact, that's what she's doing this week, as she tapes a guest appearance on NBC's ''Will & Grace.'' On the episode, which marks her first primetime acting gig in 20 years, she plays herself, with Jack (Sean Hayes) coming to work for her as a backup dancer (viewers will recall that he danced for Jennifer Lopez last season). In fact, Hayes is a decent hoofer, Jackson tells TV's ''Access Hollywood.'' ''I thought he studied dance. I really did," she said on the set. ''It's so funny. I have to catch myself because I'm cracking up in rehearsals. It's just funny to watch [the actors].''
Jackson, of course, first gained national exposure at age 10 as a sitcom actress in the late 1970s, first as the orphaned Penny on ''Good Times,'' then as Willis' girlfriend Charlene on ''Diff'rent Strokes.'' Her last regular part was as Cleo on ''Fame'' in 1984-85. After her notorious wardrobe malfunction at this year's Super Bowl, she returned to TV comedy in an April appearance on ''Saturday Night Live.'' Her ''Will & Grace'' episode will air Sept. 23.