The key to PA is definitely Philly. Gore beat Bush by 350k in 2000 and Kerry beat Bush by 400k in 2004. That is in just Philly. Dem candidate got 80% of the vote both elections.
If McCain can get 30% of the Philly vote instead of the 20% that Bush got both years then PA should flip to Mac.
If he cannot then I dont see how Mac wins PA
By improving his margin (over Bush's 2004) in Pittsburgh and western PA (which is almost a given), and by improving turnout in the "T". Also, word is going around that Obama has not provided the normal street money for Philly's GOTV operations. If that's really the case, and Obama doesn't come through, then I'd say he's essentially given up on the state.
Obviously, he would have to make up the difference in other parts of the state, especially Allegheny County and surrounding areas (Pittsburgh), SW and NE PA, and maybe some of the surrounding Philly suburbs. Bush was very competitive in all of these areas four years ago, and McCain is the type of GOPer who can probably do even better, especially with the veterans and other military types. . PA itself was a very closely contested state in both 2000 and 2004.
Again, we have already seen how poorly Obama performed in PA a few months ago. The guy just doesn't connect with many PA voters.