Clue -
I start with the assumption that seems obvious to me - US no longer wants to be in PIT. If it has to supply additional resources to do battel with LUV in PHL, then it seems most likely to draw those resources from its least desirable hub anyway - PIT. I don't imagine that they'll pull lower-cost/higher-yield flying from the hub at CLT, or focus cities DCA, LGA, or BOS.
As for the potential move to IAD. I, too, have mixed feelings about how well that could work, but I don't dismiss it out of hand. First of all, it's not between BWI and PHL. Anyone living near IAD knows it's a long miserable haul through Northern VA traffic and then DC/MD traffic before you get to BWI - and add another 90 minutes before you reach PHL. The Northern VA region has grown so much over the last decade that I barely recognize it, and I lived there for years! It's not just a DC suburb anymore, in many ways, it's its own market. Yes, much of the growth has been in the high tech arena, but that won't stay depressed forever. In fact, it's already making something of a comeback - just look at the price trends for tech stocks over the last year. It's not quite the Clinton-era go-go 90s, but money is being made. Also, beyond tech firms, the DC-area is among the wealthiest in the nation, making it very attractive for O&D. The biggest problem with IAD may be IAD itself. Though I still find the terminal beautiful (especially at night), it's design would never fly if built today. It's just a pain in the A@@. Still, on balance, I think it's a better fit for US than PIT - and IF the costs can be made reasonable - then I wouldn't be surprised to see them make the move.
As for the anti-trust concerns at IAD, I actually think Chip Munn is right on this issue. The industry has changed dramatically since the proposed US/UA merger. US is smaller. UA is smaller. The DC market is somewhat more fragmented than it was at the time. LCCs have moved into all three of the region's airports. Also - and this is big - the government now has an interest in seeing US succeed, so I would imagine that will help tip the scales in US's favor. This was a reason that some opposed the loan guarantee program in the first place - and their arguments had some merit.