US Airways will have to bite the bullet on these routes and match the fares Southwest offers. However, at some point it has to take a stand, and instead of trying to defend each route Southwest decides to add in the future, it should go on the offensive before this gets any further, and before a Southwest plane arrives at it's gate in PHL.
US Airways needs to market PHL, not as one of it's hubs. A "global gateway" is fine for international service, but what I am talking about is market PHL for the people that live in Philly exclusively. Make it truly their hometown airline. This needs to be done for CLT and PIT as well, but start with PHL since this one is under the most pressure from the LCC right now and has the most O & D Traffic.
People already know the name US Airways and see all their planes at the airport in their hub cities, but apart from that, what do they really know about US Airways? Paint the city dark blue gray, white and red if they have to. Make it's flag logo stand strong all over. Get connected with the city and it's people from small local events to large local professional sports. Create an advertising blitz from local newspapers to local TV commercials about all aspects of US Airways. Host events, parties, and donate to local communities. Create a downtown center or small store front to purchase tickets, check in for flights, and maybe even hourly shuttle bus service from it's downtown store to the airport compliments of flying US Airways. Anything to get people to embrace US Airways as part of their home town and to gain as much loyalty as possible, people will think twice about supporting a competitor into the area.
US Airways will eventually need to expand out in advertising from it's hub cities into focus cities and down from there. This all might seem backwards to you, but if they come across as the home town kid in the cities they dominate, then they can go the other route in cities they compete with. Sell the international gateway in PHL, advertise it's A330 and passport IFE. Advertise the efficient and stress free international and Caribbean connections out of CLT over the other over populated airports.
I think advertising is the key at this point, in case you haven't noticed. I wish US Airways well in PHL, it's going to be tough no matter what they do.