This would begin by mid 2013 with about 3 per month going to Delta.
http://www.marketwatch.com/story/southwest-airlines-delta-air-lines-and-boeing-capital-reach-a-tentative-agreement-to-sublease-airtran-boeing-717-fleet-2012-05-22
http://www.marketwatch.com/story/southwest-airlines-delta-air-lines-and-boeing-capital-reach-a-tentative-agreement-to-sublease-airtran-boeing-717-fleet-2012-05-22