I just made my reservations on May 9th. I got a flight for 9/20 return 9/25 with US air for $168.50, I actually booked this through United's website because they had the price I wanted and the times I wanted. The USair website had the $168.50 price but the flight out of PHL was not until 12:30 and that is a waste of my first day, since I would not get to my hotel until around 4:00pm.
I generally use USair I find that if you search for price rather than schedule you can do pretty well. Last year I flew in December and the cost was $164.50, I had booked that in August.
I have the 10:30 am flight out of PHL and the 4:30 pm flight out of MCO, when I checked on this flight the other day it was up to $265.00 roundtrip.
You really have to check every day, sometimes more than once a day as the prices change so frequently.
AirTran flies out of PHL and I have heard a lot of good things about them. They have none stop flights and the flight times look pretty close to what USair runs.