This is beginning to sound like a plot for a premium channel series about rivalry in the entertainment biz … Showtime vet Mark Greenberg could wind up running the studio joint venture that plans to launch a new premium TV channel, VOD and online service in 2009. Announced Sunday, the ambitious JV includes Viacom (NYSE: VIA) and its Paramount units, MGM and Lionsgate. Viacom’s exclusive pay-movie deal with Showtime expired at the end of 2007; as the story goes, Viacom wanted more to extend, Showtime wanted to pay less, Viacom is taking the ball to another field — and has recruited two studios with exclusive Showtime deals set to expire. Now BusinessWeek reports that Greenberg, who also worked at HBO, is expected to be appointed CEO, who their sources say was forced out in 2006 after clashing with Showtime CEO Matt Blank. Viacom isn’t commenting on this or any of the other stories flying around, a popular theme being sibling rivalry between Viacom CEO Philippe Dauman and CBS (NYSE: CBS) CEO Leslie Moonves, whose Showtime takes the biggest initial hit from the new JV.