how many people who swallowed this story whole will alter their opinion of Palin now that it's been proved false?

My guess - almost none. They believed it because they see it as consistent with, not contradictory to, their impression of her.

So as far as the political fallout for MSNBC goes, I'd say - little to none. The lack of fallout should be sobering, but most people won't even notice, because apparently this stuff only matters when it's perpetrated on a member of the home team.

This is just one example of how partisanship undermines rationality, and how we fail to attend to things that matter because attending to them might undermine the consistency of our belief set.