Most mainstream media sources make a big deal about being "unbiased", but none of them succeed, and it's to the point where I think it's time we all admitted that objectivity is an unreasonable goal. Instead of feigning objectivity, journalists should just be honest about their personal biases and allow their audience to take them into account.
It's possible to be biased and still be honest, but it's impossible to be honest while pretending to be unbiased. I don't doubt that many/most in the media try to be objective, but it's clear from public opinion polls that despite their efforts they're failing miserably. One thing I like about bloggers is that we generally admit our biases for the sake of honesty; we do our best to get our facts right, plus we disclose our prejudices so that our readers can factor them in. Everyone who reads this blog knows that I'm a Christian and a Republican, and I'm sure you can see the influence of those two facts (as well as others) on everything I write.
The problem with the mainstream media is that they've built their reputations on the idea that they're objective. Sure, polls show that most Americans know that most journalists are incredibly biased (just like everyone else), but journalists have to maintain the delusion of objectivity because they use that facade to inject their opinions into the public debate. Without the illusion of objectivity, most of the opinions of the leftist media would hardly be worthy of discussion by rational human beings. (See, I'm biased.)