... or what (not) to watch for in the primary results.
Contrary to most states in the US, Pennsylvania usually gets results from the larger metropolitan areas followed by more rural results later in the evening. This means it's quite possible we'll see Obama doing well early on due to Philly results, then Clinton will slowly pull away as the night goes on. This isn't a prediction, just something to keep in mind as results begin to come in.
Additionally, exit polling should not be considered an indication of the outcome. It should be reviewed after the fact to understand who voted for who, but is not likely to tell you who will be victorious or what the results will be.
Lastly, here are some polling images from FiveThirtyEight.com showing the breakdown of support by congressional districts from the latest polls available:


How Obama does in the southeastern part of the state will determine how he does overall. If his campaign has driven enough new voters to swing greater than 60% wins here he'll have a very good night (Note: I'm not saying he'll win, but I don't think he needs to 'win' to have a good night. ;)