I think one thing that contributes to that is how vocal some members of the media are concerning their political views, and you don't see conservatives very often. They also make it obvious many times with how they word their articles and headlines.
It's like the old joke Jesse Jackson told when he hosted Saturday Night Live. He said that he had an audience on the Pope, and he and the Pope and all the rest take the ferry on the tour. As the Pope is standing at the railing, the wind blows his hat off and into the water.
Jesse says "Don't worry, your Holiness, I'll get it." He jumps over the side, walks on the water, picks up the hat, and walks on the water back to the boat. The Pope is amazed and everyone congratulates Jesse on the miracle.
The next morning the headline is "Jesse Can't Swim!"
You can read much of the time how stories are skewed, like today when CNN had the nerve to say "Cheney has been cleared of wrongdoing by authorites... but questions still remain." It's obvious they are milking it. If you watch where stories get put in, say, the New York Times, you'll find the preponderance of stories vindicating Bush don't end up on page one.
Sure, they always gang up on whoever is in charge, but the fact that they don't hide their personal biases when they are off camara makes it easier to spot how biased some of the statements in their stories are.