Yeah, while this seems to have some weird geeky appeal, it's still way harder than just pulling out a dollar and handing it to someone.

I'd say the novelty of it is enough of an incentive for such an idea. Also, bumping two phones together carries lower psychological cost than handing someone your hard-earned cash and I would expect people to have less qualms about lending/returning/trading small sums of money this way.

The api will be free for most people. If you use it for revenue generating transactions or it is core to your app (wouldn't be useful without it) we ask for a revenue share (but no up front fee). Otherwise, as long as you don't hit our generous usage limits, it is free. The one exception is that contact sharing apps aren't allowed yet, although we have a plan to allow that in the next release.