With Mel Karmazin's abrupt resignation yesterday from Viacom, all the talk has been over where he would plop next with his considerable talents.
   But for media buyers the more immediate concern is what his departure might mean for CBS.
   Under the new order announced by Viacom chairman Sumner Redstone, CBS's Les Moonves and MTV Networks' Tom Freston will serve as co-presidents and co-chief operating officers of Viacom, reporting directly to Redstone, each with expanded management purviews.
   Moonves will continue to run CBS and the station group but also inherits the radio and billboard operations, which have been sagging, as well as Paramount Television.
   Freston will continue to run the MTV Networks but will also oversee Showtime, Simon & Schuster, Viacom's book unit, and Paramount Pictures.
   In the short term, concerns among media buyers regarding CBS are minimal, certainly as the upfront market gets underway.
   "There will be no effect on the marketplace," says one media director, "because the marketplace isn't run by CEOs or COOs. It's economic supply and demand, and half of their business is already done." 
   Longer-term, CBS's fate is more problematic, and here perhaps Moonves' biggest challenge, as he takes on additional responsibilities, will be to keep CBS free of the sort of interference that has hurt both ABC and NBC over the years, when corporate chieftains stooped to impose their programming savvy--or more often their lack of--on the creative minds responsible for coming up with new shows.
   Caught by surprise by yesterday's announcement of Karmazin's departure, media people were reluctant to speak on the record about their concerns for CBS. As one media director put it bluntly, responding to a query from a Media Life reporter, no one is going to talk on the record about the Viacom changes, at least now.
   But the sense calling around was that there's some trepidation about changes that could come with Redstone's consolidation of power. 
   "The Viacom people were doing pretty well on their own, but I think a lot of people are just waiting to see what happens over the next few months," offered one top media buyer.
   Some on Wall Street echoed that concern.
   "We view this announcement as a very significant negative for Viacom as this represents the loss of an extremely talented operating executive," said Merrill Lynch analyst Jessica Reif Cohen in a note to investors yesterday morning.
   Cohen, a longtime Karmazin supporter, made special note of the departing executive's willingness to buck Redstone.
    "Mel Karmazin is talented and importantly, a very strong executive, capable of saying 'no.' 
  "Although we believe that the company's remaining executives also possess a keen sense of financial discipline, we view Mr. Karmazin's ability to hold firm as unique." 
   Just what the chances are that Redstone will begin meddling with CBS is a big unknown.
  Arguing against it are several major factors, and not the least is the huge success of CBS in whittling away at NBC dominance under Moonves and Karmazin. Even the most meddlesome top executives are reluctant to interfere in operations that are running smoothly.
   Moreover, the conflicts between Redstone and Karmazin were personal and had almost entirely to do with control over the destiny of Viacom, far less to do with how the conglomerate was managed under Karmazin in his role as chief operating officer. Redstone was never one to cede operational control to another executive in the years he built up Viacom.
   Further, with his adversary now out, Redstone must prove to a doubting Wall Street that he can manage through his chosen lieutenants. And that means letting them do their jobs.
  Another argument against Redstone's meddling is the personality of Moonves, a hands-on and feisty manager in his own right, one strong enough to work for the relentlessly demanding Karmazin. He's no pushover.
   But all that said, there's no telling what Redstone will do now that his path to total control has been cleared. He's unpredictable, irascible and used to getting his way.
   And that alone opens up all sorts of possibilities for interference in the running of CBS.
   That could mean lots of trouble for CBS. 
   ABC and NBC have both endured such meddling, resulting in a free fall for the former and a gradual weakening of the latter. 
   Indeed, in the opinion of many media folks, the rise of CBS had less to do with its improved programming than with the decline of NBC with the departure of talent to other networks. 
   The network has paid a hefty price with the steady decline of its dominance on Thursday night and the lack of a breakout hit comedy since "Will & Grace." 
   Under the Disney regime, ABC has had a rotating door of executives, 13 in the seven years since the merger. Each one has had to deal not only with a mess of a schedule, but an unclear chain of command that ultimately led to Michael Eisner and made it nearly impossible to tell who was really making often disastrous programming decisions.
    CBS is now considered the most stable of the networks this year, having a schedule media buyers and planners recently voted tops for the fall in a Media Life poll.
    Under Moonves' direction, CBS has won six straight sweeps periods among households and total viewers, and finished No. 1 this season in adults 25-54 for the first time in a year when CBS did not carry the Olympics since 1981.
  Moonves' challenge will be to keep that momentum without Karmazin behind him.