Is there actually evidence that higher poll numbers in favor of X lead to higher voter turnout from supporters of X? It seems like everyone takes that for granted but I've never seen any evidence that it's true.
I'd like to see some raw evidence, too.
But since we're forced to speculate at the moment, my bet would be that the relation to voter turnout and a candidate's chances of winning isn't linear. It's probably more of a parabola: the more extreme a candidate's chances of being elected (or being defeated), the less likely anyone is to come out and vote, because it feels impossible to make a difference. On the flip side, I'd suspect that the closer and more contested a race is, the more likely people are to feel obligated to vote.
Under this theory, polls/predictions can't actually skew the result in either direction [1], they can only increase or decrease the total turnout (highest turnout when polls say 50%, lowest turnout when they say 0%/100%). In reality this probably isn't actually true, but we can't say for sure whether or not polls do affect election outcomes, and in which direction, unless there's empirical evidence. I don't even have a guess for which direction it would go (whether you want your supporters to be "concerned" or "optimistic") - I could see either being true.
[1] Edit: I should note that this assumes that each poll/prediction is listened to and taken seriously equally by both sides of the electorate, which almost certainly isn't true. Which actually brings up something kind of interesting - it could be that with all of its "Romney will win in a landslide" talk, Fox News actually hurt Romney's chances because most Fox News viewers are Romney supporters (but they could have done the same amount of damage to Romney's chances by saying "Obama will win in a landslide" - the best way for them to help Romney might be to say that the election is exactly tied). And I suppose it would also lead to a justification for the idea that Nate Silver has a liberal bias, using his relatively "wishy-washy" predictions to energize his mostly young and liberal audience to get out and vote. It seems that I've gone too far with this and started arguing against myself...
However, in a winner takes all election, the effect would have to be huge. Let's say all polls indicate a 51%-49% result. Then, at least 4% of the winning party's voters would have to stay home 'because they already won' to change the result (and that assumes none of the other voters stay home 'because they already lost'). At a more realistic 60%-40% poll prediction, one in three voters would have to stay home.
I have no evidence for this whatsoever.
I think my point still stands though: this doesn't showcase NPR's neutrality - perhaps they are in fact biased toward Obama and just lie more strategically than most Republican pundits. (Certainly not accusing them of that, only saying that I don't think we can glean much from the fact that they gave Romney higher numbers than he deserves.)
I'm not sure it's really a question of ideological bias so much as filtering the raw data through a backward-looking model.
The former can occur simply because of the assumptions that your model makes. Bias in "statistical bias" does not mean the same thing as "political bias".