PIT could be a good base for a start up if CMH can work for Skybus then why not PIT for a new low cost carrier. Of course, Skybus had very good solid financials and took years to get off the ground. I doubt they'd add PIT as a focus city though.
The only other new carriers that I have been hearing about for that long was Virgin America and Air Gumbo. Virgin America has of course since started flying and I doubt they will add PIT as a hub. Air Gumbo was planned to be based out of MSY but I think the guy planning that venture has had trouble finding investors and again I doubt PIT would be a hub for them If they ever start flying. If any other investors decided to start a new carrier and base it out of PIT it would probably be years before they started flying, if it were to be a sucessfull venture and well funded. Of course someone could always throw something together overnight in conjunction with some charter outfit like Pace or Xtra Airways ala Hooters airlines but they were unreliable ultimately because they could not afford to pay for fuel when it suddenly increased and they cancelled flights suddenly.