Mark Blumenthal has a super-helpful post sizing up the differences in demographic composition among the various Pennsylvania polls, along with this useful chart summarizing all the crosstabs. The upshot (which I'm not sure Mark would feel 100 percent comfortable with) is that Obama's probably going to have a decent night (say, 7-point loss or less) if the African-American share of the vote hits or exceeds the upper teens, and if younger voters (ages 18-44) get into the low 40s or higher. If those demographic groups don't hit those thresholds, I'd guess Hillary's going to win by close to ten (or more) points.
(And, obviously, Obama needs to overwhelmingly win African American voters, do well among younger voters, and not terribly among white voters. But you already knew that...)