AirTran serves Houston Hobby to Phila. and Dallas to Phila, both with connections in Atlanta. It's possible that other airlines will match AirTran's fare, sometimes even non-stops such as US Air or Continental out of Houston IAH, or American or US Air out of DFW.
That doesn't mean that there wouldn't be specials run from Austin though. It would be determined by your travel dates. It seems that the fare would have to be substantially cheaper to drive all the way to Houston or Dallas. I would just check a lot of sites to compare prices, and subscribe to the fare tracker emails from Travelocity and Expedia.