Southwest's legions of loyal flyers take note: Your cash is worthless onboard. The airline will begin accepting only credit and debit cards for all onboard purchases September 9. Southwest doesn't actually sell a whole lot onboard—mainly alcoholic beverages and energy drinks—so this move makes sense from a "keep it simple" standpoint.
Southwest is also likely hoping the convenience of paying with a credit card boosts onboard sales a bit. Makes good business sense, to be sure, though the airline will have to pay some sort of fee to the credit card companies. It will be interesting to see if prices change to account for this. That said, I'm sure many folks (myself included) would succumb to the urge for an onboard beer if they weren't so often flying cashless (like I usually do, and not intentionally). So by switching to credit cards, Southwest is taking away the "no cash" excuse, making it easy for passengers to give in and buy a drink.
Just don't pack your credit cards in your checked baggage.