Not sure if I'm a particularly niche market, but I'm really jonesing for something that lets me use my mobile as a payment card.
I run longish distances for fun, 15 or 20 miles on a Sunday morning. Sometimes, rarely, though sometimes in the summer, I fail to plan for the conditions, get too hot, need to get a drink, or simply bonk and need to get home.
For this reason, I always carry a phone which contains my ICE details. I can phone the wife and get her to come and get me. I don't carry a wallet with all my cards in. The phone has a radio and a camera so I can use the one device for all my needs except paying for stuff.
If I could pay with my phone for trains and buses, not only does this offer me emergency payment for refreshments/to get home, but actually opens up a lot of new routes for me. I could, for instance, get the train down to Market Rasen in the morning and run home.
I'd imagine there are other endurance athletes who would also welcome this. Cyclists would potentially be a big market as a large part of their activity involves the purchasing of tea and cakes.
I'd love to see this implemented.