A company in Switzerland called Fairtiq has an iOS app with support for the Apple Watch that lets you pay for transit tickets using your Watch.
The way it apparently works is you start the trip on your Watch as you board the bus (they also have partnerships with rail carriers), and end the trip when you leave the bus. The iOS app automatically calculates the fair and pays for it, based on geolocation of the start and end points.
An article I found also says that if you take multiple trips in a day, the app is smart enough to buy you a day pass instead if the price is advantageous to you. I would use this in a second on New Jersey Transit on the way to New York if it was supported. [ via The Loop ]