I don't believe that NW is that interested in PIT, PHL, or CLT. There has been very little interest as of yet and over the past year they actually reduced capacity in these markets. If USAir does go under I'm sure they will bolster their capacity to into the hubs to serve the local market but I doubt they would make them hub cities. USAir already proved that these cities aren't gold mines.
Any new Airlink will most undoubtedly fly along side of PCL and Mesaba out of MEM, DTW, and MSP (and possibly MKE and IND with PCL). It will be interesting to see who is going to be willing to take on this contract if NW ever offers it. NW executives are control freaks and they won't stand for truly "independent" operations under their banner. Any new contractor will most likely have to completely sign over control of that aspect of their company.
Anyone who has ever flown under NW colors knows that its their way or the highway but then desperation will likely be high among Mesa, CHQ, TSA, and any of the WO's should they escape the liquidation.