Southwest Airlines is going plastic-only starting September 9th:

Southwest Airlines wants you to keep your cash ... and hand over your plastic! No, Southwest is not adding new service fees, but beginning Sept. 9, all Southwest Airlines aircraft will be equipped with handheld credit card devices that will enable Flight Attendants to take drink orders in a flash, and accept credit and debit cards for payment of cocktails, beer, wine, and Lo-Carb Monster energy drinks. The airline will no longer accept cash onboard.

You may remember that Alaska Airlines just did the same thing. And US Airways has said it's their goal to go cashless next year.

Soon your cash will be worthless when airborne.