I've been monitoring television news programs a little more closely than usual. Recently I read an article by Jay Rosen, a famous journalism professor/critic at NYU, who gave examples of how the TV newsfolk determine what is acceptable opinion by excluding opposing opinions from televised discussions.
A couple of good examples were the run-up to the war in Iraq and the more recent healthcare debate.
Someone, it seems, in late 2002 and early 2003 had made the determination that an invasion and occupation of Iraq was the right policy for America. Almost immediately, any dissenting opinions were marginalized, kept off the air, and ridiculed by pundits, if they were mentioned at all. This despite the fact that a sizable portion of the population had doubts about the "evidence" of Iraq having weapons of mass destruction.
The rare few who gave air time to people holding such opinions like Phil Donahue, in short order found themselves looking for work. And this wasn't just right-leaning Fox News, but all the networks liberal-leaning MSNBC included. Newspaper and magazine columnists who supported the invasion wholeheartedly like Thomas Friedman of the New York Times, Jeffrey Goldberg of The New Republic (then of the New Yorker), and Jonathan Alter of Newsweek, have seen their careers and influence flourish while those who expressed doubts (and were proved right), like Robert Scheer, of the Los Angeles Times, had their employment terminated and were basically ignored in the debate.