It looks like AirTran out of BWI does beat the US Air price out of PHL on most days. I actually live only 20 minutes from PHL, so it's hard for me to justify travelling to another airport just to save some money on the flight ticket. But its great to have some other options.

I have found that you get the best the US Air prices for round trip PHL to MBJ if you book Wednesday to Wednesday. Most of the time, the price is comparable to AirTran.