[jonsmirl] and others suggested AoA -- which mostly seems to be an Android-side API to interface with USB devices.  Not a bad idea, but what do we use for hardware?

Well, there's the official dev kit which doesn't exist anymore.  Google/Android now wants you to tether the phone to a Raspberry Pi.  LOL, thanks.  The whole point of a cellphone/hardware interface was replacing the Pi.

Arduino made an ADK board for a while.  Looks to be discontinued.

It looks like you can still get the IOIO board from Sparkfun, so that's good news.  It's $40.  Bummer.

So DIY? It looks like AOA needs a USB host attached to it.  So you'd better like coding up USB host firmware.  (I don't.)

I haven't looked hard enough into AOA -- If I'm missing something here, please let me know!