Turn out is always important in elections. But I don't think that the GOP is really going to get great turn out either. People talk about how people will show up to vote against Obama. History shows that really doesn't happen that much.
The thing people forget when just looking at pure polling data with respect to who are you going to vote for (i.e. who are you going to vote for?) is Romney's negatives. It is VERY hard to win an election when people say they dislike you as much as they do him.
You almost have to imagine a scenario where people are saying they dislike him, but they don't.