Santorums job performance: Approve 48% Disapprove 46%
I assume that's a part of this same poll?
If so, you could spin that either way. I think it could be said that if Santorum's approval rating is plus 2 but he's still 10 points down, then obviously Casey isn't as horrible of a candidate as people are making him out to be. It would seem people who like both men are largely voting for Casey.
Or it could also be more evidence that the national environment is dragging Santorum down. Voters may be punishing Santorum for their dislike of the President.