I looked into the Raspberry Pi (and Arduino, of course) but the Android+IOIO is currently the cheapest and simplest way to implement a wireless connection (IOIO is $45 plus a bluetooth dongle for $5-10).
The controller is communicating wirelessly with my phone via bluetooth, so theoretically your app could do the same thing. Once I get things running the way I'd like I can look into adding some sort of plug-in so that external apps have access to the data (or maybe help you write a plug-in for your own app). The code is at https://sourceforge.net/projects/udssupervisor/
if you're curious.
What would be cool is someone with experience designing circuit boards could create one using my design that could then be ordered online. Then people could order a board and parts; the hardest thing would then be just soldering a few components to the board.