My guess is that the increased capacity at PHL is coming from a variety of sources: mainline jets freed up by Midatlantic taking over former mainline routes (for example, PHL-IAH is all-MDA now), additional MDA/Express flights (PHL-MCI goes from 2 MDA/1 Express to 4 MDA in the new schedule), and increased utilization. It looks like most of the new flying is done by mainline, though I haven't looked at other markets to see how they fare come October.
To me, it seems pretty clear that much of the added capacity is aimed towards non-stop WN (or FL) markets at PHL (FLL, MHT, MCO, PVD, RDU, TPA, PBI -- 13 of 18 flights) or potential non-stop WN (or FL) markets from PHL. It's somewhat good that US is moving to protect its hub franchise, but only if they've figured out how to make money at WN/GoFares fare levels. At least most of the new flights seem to be on mainline equipment.
Addendum:
Protecting market share is all well and good *if* you can make your competitor bleed enough for long enough to convince them to pull out of the market. It seems to me, though, that WN intends to be in PHL for the long-haul, and US needs to plan accordingly. At least in the short term, WN won't have the gates to respond to US's added frequencies.