The Times and the Post will repeat the McCain camp's petty lies and slams without an ounce of critical analysis, not because they believe the spin, but because there's a massive conservative infrastructure devoting to working the refs and getting the coverage the right wants. Try as David Brock and John Podesta may, Media Matters and Think Progress just don't have the same level of influence. I absolutely guarantee that if the Ron Fournier situation were reversed - if the AP Washington Bureau were run by a shameless Democratic hack instead of a GOP hack like Fournier - there'd be hell to pay. Howls of "liberal media bias" would be echoing across every talk radio station, the major conservative blogs, Fox News, and into the mailboxes, real and virtual, of every Republican voter. The bureau chief would be gone or tamed within a week or two. As it currently stands, Fournier still has his job, and he's still doing it as badly as you'd expect. Liberals just aren't as good at ref intimidation.
Part of the reason why this is true is that the Right realizes that it isn't enough for the Media Research Center and Accuracy In Media and the Heritage Foundation to engage in media criticism. They all do it; it's a staple of the entire conservative movement. So the refs get worked every day, by a wide variety of people -- from local activists to the President of the United States. The Republicans are all media critics.
Progressives understand this much better than they did a few years ago, but there's a long way to go. (Ezra Klein is doing his part: his writing about the media's coverage of the presidential campaign has been excellent lately.)