In response to my original bold proposal post, Mock raised the question of whether or not a Phillies/Pirates rivalry actually exists. As luck would have it, I know quite a few Pennsylvanians - most of them Phillies fans. So, I posed the question to them on facebook and the consensus was that there is no rivalry. Phillies-Mets is a much bigger deal. According to my sources in the field, Pennsylvanians either like Philadelphia or they like Pittsburgh. It's not really about hating the other.
Almost all of my experience in the state is in the central region. My family has a (very modest) summer cabin not too far from State College, home to the largest Penn State University campus. If any town in the state is regionally neutral, it's State College. While Pittsburgh is technically closer than Philly, the student body obviously draws from throughout the state so if one walks into a grocery store/Target/Wal-Mart, one sees products available with the sports logos of both cities. In fact, I would say that the prevalence of one team over the other has more to do with their relative prowess - i.e., if the Steelers win the Super Bowl, you'll see more Steelers stuff. If, on the other hand, the Eagles had a better year, there's far more green and silver. It's been a long time since the Pirates were any good, of course, but my bet is that if they had a resurgence, the bottle opener availability would turn in their favor.
One must acknowledge, however, that in State College, all teams rank well below Penn State football, as do sunshine and puppies.
My point in bringing up the prospect of a Phillies/Pirates rivalry is the fact that Major League Baseball has always promoted the idea of inter-league play as fostering regional rivalries: Yankees/Mets, Indians/Reds and so forth. But perhaps the non-rivalry in Pennsylvania is a good demonstration of the fact that familiarity does not necessarily breed undue contempt. The two teams have both been in the National League for, get ready for it, 123 years. They've both been in Pennsylvania the whole time, too.