Strange Doctrines has a post on the subject that says a lot of what I tried to say in an earlier discussion, but says it much more coherently. Here's a key quote:
Do mainstream media exhibit a reliable political bias? The vagueness and complexity in the question as asked leave me without a settled opinion. In any case, the the claim that media have either a "conservative" or a "liberal" bias simpliciter (even assuming that the terms media, conservative, liberal and bias are well defined) is an incredibly strong claim that is itself politically charged and obviously subject to the political bias. Getting a handle on what objective phenomena would be entailed by such a hypothesis would be a pretty serious undertaking ill served by impressionistic musings, glib inferences drawn from cherry-picked examples, and methodologically suspect studies.