I know that's a vague title above. Humor me.
There is a piece in the latest Public Opinion Quarterly that does a nifty job of demonstrating the effect of media coverage on nonresponse -- basically the tendency to say "don't know" or "unsure" in a survey. The more coverage by the news media, the less likely people are in a survey to say they don't know or cannot provide an answer.
The authors, Stroud and Kenski, place this in an agenda-setting framework. For those who missed mass comm grad school, agenda setting is the notion that the media do a lousy job of telling people what to think but a helluva job of telling people what to think about. Agenda setting is not a theory, more of a serious borrowing from priming and accessibility theories out of social psychology. So yeah, agenda setting suffers from a "so what" problem. This paper provides one of those "so whats" and it's a nice job of doing so.
What's the link to what people know?
Think of the present presidential campaign. Yes, I know, it hurts to do so, what with Clinton's campaign using kindergarten papers written by Obama to push polling against McCain, possibily by Huckabee's folks. It's a sad, tired, bunch of people running for office. Power attracts the corruptible, as someone once said.
What the media covers influences the likelihood to answer a policy question. That influences in a subtle way the findings on that policy question. This influences the candidates. This also colors coverage by journalists, who for the most part are baffled about polls in the first place. If hearing or reading about something in the news makes you more likely to provide an answer, this also affects political knowledge, the ability to correctly spit back to a pollster some response to a survey question.
Nonresponse can matter in polls. The influences are not huge, but they are enough to matter. And apparently the media can influence the likelihood to give an answer, or to pass.