Matthew Vanitas recently started a thread polling for folks who might be interested in buying a Hayden midi concertina:
http://www.concertina.net/forums/index.php?showtopic=16394&hl=
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 Ćukasz's last post here:
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.