You're absolutely right - I agree that voter turnout "should" be highest at exactly "50% chance my guy wins" and diminish from there in either direction. I think I just got a bit too caught up in what I was arguing when I mentioned that a 10% Romney prediction should be the best one for Romney supporter turnout; a 90% prediction should probably be just as good as a 10% prediction in terms of making you likely 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...
There is no guarantee that the effect of an almost certain victory/loss is identical for all parties. On the contrary, there is some data that indicates that supporters of some parties are more motivated to vote. See for example http://www.ncbi.nlm.nih.gov/pubmed/22065127, which shows that weather conditions can affect election results. I cannot find data on it, but I think it is not inconceivable this extends to 'going to the polling station even if it does not make a difference to the result'
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.
If you want to go full on conspiracy mode, you could argue that Fox news' owners wanted Obama to win due to the belief that 4 more years of being able to lament the horrors of a having a Communist Muslim in the White House would be much better for their ratings. And that is why they pushed the Romney by a landslide thing so hard.
Just blowing smoke rings here, but it also could be possible that "undecideds" might break towards a perceived winner. If they aren't committed, they may just pick the winning side.
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...