Spirit announced they will fly one PHL-DFW flight beginning in April.
http://www.myfoxphilly.com/story/190...illy-to-dallas
For those who haven't yet flown Spirit, I can confirm that it's unpleasant. But not horrible. Everyone, including me, apparently has their price -- at least if they're paying for their own ticket.
I suspect US may match some or all of Spirit's PHL fares. They pretty much did this on Spirit's DCA-FLL flight. (Spirit recently decided to move that flight to BWI.) So I would look for some PHL-DFW bargains next year. I see Spirit is currently offering a sub-$40 one-way fare on some flights in that market, but it's only good on 1 flight a week (they will start with 4 and go daily by summer).
Frankly, from a business perspective, this seems like a horrible market for Spirit to enter. Spirit's business is leisure travel. Folks travelling on a business trip (and that's probably most folks travelling between PHL and DFW) won't fly them. I certainly wouldn't. So what's the point? This DFW expansion strikes me as, perhaps, the beginning of the end for them. We'll see.