If my memory serves me correctly, Boeing owns the paper on Air Tran's 717's. Boeing surely won't let SWA out of those leases unless another carrier agrees to take them in lieu; they would be a tough lease/purchase for Boeing to negotiate in this market. On the other hand, DAL may be able to secure a preferable rate or price on the 717's when considering the colossal failure of the 787 and the financial impact to DAL caused by the Boeing delays. In all reality, those 717's hold little value to the other US major airlines and the rest of the world has long gone the route of 737/320.
If what I have heard is true, the 717 shares commonality with the Saudi MD-90's which might make for a common fleet type and a fleet large enough with sustainable economics well into the future. The last results I heard of showed strong sustainability with the MD-88 at $3.50 gallon over ten-year leases on 320/737's.
It would be interesting if DAL would consider updating the existing or longer-term 88/90's to the newer cockpit configuration. I buddy of mine claims $2 mln a copy to convert an 88/older-90 to glass.