I'm working on a hardware device that will plug into the USB port of a mobile phone and transmit data from one phone to another at very high speeds. The data is being carried over visible light (so it would appear that you are plugging in an external flashlight to your phone, except that it's carrying data). Data transmission for the first prototype will be 10 Mbps, later prototypes will be able to reach 100 Mbps.
I think this opens up some interesting possibilities for new app development, since you are now providing a low latency, high speed connection between adjacent devices. The fact that the user will be able to see where the data is going also provides some potentially useful feedback. The immediately obvious apps are for things like fast file transfers (pushing a song from one phone to the next), gaming (imagine playing blackjack where a "dealer" points their phone at other phones and deals cards just as they would in real life), and mobile payments (light transfer is private).
I'd be interested to see if this is something the developer community would be interested in getting their hands on and what sort of applications come to mind.