Update: This offer is expired. You can find the current offer details here.
Southwest is offering a 15% off code for those with a Signature Visa credit card here. Any card which says “Signature Visa” on it will qualify (for many of us, that would be the British Airways Signature Visa), and it is good for any “Wanna Get Away” fare. The ticket purchase must be made by April 30, 2011, for travel by June 1, 2011.
(Tip of the hat to MilesQuest)