Once all the hullabaloo settles down, Palin's selection changes very little - to the far right, she's a gun owning, God fearing defender of their beloved 'traditional' American family values. But those people were never going to vote Dem anyway. She may rescue a few votes from independents and Libertarians though.
To the far left she's a typical right-wing religious fundamentalist nut-job who'd replace schools with bible classes and who's own family can't live up to the ridiculous moral standards she herself preaches to the rest of us. Those people were never going to vote GOP either. On the other hand, people in the middle ground who were maybe swaying towards McCain's more center-right politics might be pushed back towards Obama.
As I say little will change. Forget McCain, Palin and Biden - in a week or two it'll all be back to where we were before the conventions: a decision to vote for, or against Barak Obama.