Sustainability. Sexy. Two words you don't often see together. Sustainability is a serious word. It's about saving the world from ecological disaster. Getting humanity on track for survival. Heady stuff best left to academics, unions of concerned scientists, and earnest tree-huggers. Would you buy this? Photo: iStockphoto/Tyler Stalman. Sexy ... now that's a fun word. A word that implies pleasure. Tied to advertising, it's a hook that's been used to raise sales curves on everything from cars to cognac. A very big opportunity awaits if we -- marketers, producers, and media -- manage to somehow join these two words at …