From a USA Today/Gallup poll:
Two of three registered voters, 67%, say putting Palin on the ticket won’t affect their vote; 72% say that of Biden. Of those who say the running mates will make a difference, 18% say Palin makes them more likely to vote for McCain, 11% less likely. That net-positive impact of 7 points is similar to Biden’s: 14% said his pick made them more likely to vote for Obama, 7% less likely.