Besides the Bradley/Wilder/Gantt effect, pollsters have not been as good at polling rural voters as urban and suburban voters. Of course Obama is hoping for young voters to turn out better than they have historically and to have cell phone only voters who are not measured by the polls. Still, I suspect the rural voters favoring McCain/Palin by a greater percentage means Obama will not get as high a percentage of the actual votes as he gets in polling.
A little harder to rig the vote to match the polls in rural areas too.