Having gone to school in Baltimore and a native of Pittsburgh, I can pretty much tell you you're going to need to fly into Pittsburgh or rent a car. BTW, why does the topic say Harrisburg if you need to get to Pittsburgh? Are you interviewing at Penn State or Pitt?
If you're going to Pittsburgh, the best thing to do is fly to Pittsburgh and then take the bus from the airport to Oakland where Pitt is located. You can usually fly USAir into Pittsburgh, we're a USAir Hub or find something cheaper on another airline. If you can't fly into Pittsburgh and insist on going to BWI, there really aren't too many options to get from BWI to Pittsburgh other than another flight (~$100 roundtrip) or an 8 hour bus that goes to DC first and then Pittsburgh. AMTRAK doesn't run from Baltimore to PIttsburgh.
If you're going to Harrisburg for Penn State, I think you can take a Super Shuttle from BWI to Penn Station of the bus station and either will take you up to Harrisburg. Greyhound will cost you about $45 round trip and the train maybe $20 more.
Good luck!