It seems that the Clinton media is framing the PA race as the most important race since...well, ever. And if it runs true to form, Obama will start out behind, close the gap quickly until it looks like he might win, and Hillary will start throwing mud.
Obama will get enough delegates to mitigate losing PA, but it seems like that counts for nothing in the media. And no one will care if he starts 15 points back in PA and can close the gap to 5 pts or so and still keep near parity in delegates.
Obama needs a home run to salt this away, change the dynamic, put away PA and Hillary for good and here's how I think he can do it:
Offer Edwards the VP position now. If he accepts, announce it in PA and get him out campaigning as a team, just as Bill and Hillary are. The Obama/Edwards ticket will claim the voters that previously went to Edwards as well as make a huge dent into HRC's better demographics. This will also set up NC nicely, but I doubt it gets that far, because I think this will change the campaign overnight.