I am afraid that I have hijacked his thread by starting to talk about doing some sort of homebrew, DIY, midi concertina project. Let's move the discussion about a DIY midi concertina from Matthew's thread to this one and leave Matthew's thread to its intended purpose.
So, to reply to Łukasz's last post here:
There is one problem with iDevices - you have to build a true standalone MIDI controller based on Arduino (or any other microcontroller), which sends MIDI messages over a MIDI cable and this is a lot of computation for Arduino (and my board had too few inputs and outputs to make that happened). The reason for that, is that iDevices require deviceID for anything other than MIDI accesories and this is not easily obtainable. At least that's how it was three years ago, this could have changed. That is why I wrote about Android devices, which have no such limitation. But yes, a smartphone as a "brain" for such concertina is probably the most versatile solution. But it is a path towards commercial production, not a single, DIY controller.
In my concertina Arduino serves only as a gatherer for different values, which it then sends in raw form to Processing driver running on host system. The reason for that is that Processing has some neat MIDI libraries, Arduino does not. This requires a host capable of running my code and support for a software MIDI device. Back then that was true for OSX, Linux and Windows.
If you were starting over from scratch then would you use an Arduino/Shield now, or would you use a Raspberry Pi instead? I don't think that RPi was available 3 years ago when you started your project. Maybe keep using an Arduino as a device controller and use a RPi to run the Processing driver and generate midi o/p.
A lot more power and capability in an RPi including some synth libraries so it might be possible to use it for an in-concertina synth.