They're piloting self-serve ordering kiosks at the Nats Dog stand behind section 142. Seems like a good idea but I can somehow see it becoming a disaster when they get busy.
They have self-serve beer kiosks at Great American Ballpark in Cincy. I can think of about 11 ways for that to go wrong but didn't actually see any issues when I visited.
There's a local bar (Meridian Pint in Columbia Heights/Petworth) that has self=serve taps in the tables. You give the waiter a credit card, they turn on the taps, and it meters your pours just like a gas pump. Dangerous in more ways than one!