I'm everything but a long term forum member ( as a matter of fact I only registered today after lurking a couple of weeks) but I must say that this shield looks very nice. Is this the one you mentioned in your arduino project blog?
I've been experimenting a bit with the auduino granular synth myself. I learned about the arduino as I was googling for a way to retrofit a bugbrand Workshop Oscilator Machine with MIDI. I will definitely follow the evolution of this project.
Edit: I noticed that this design only mentions a MIDI IN where your blog mentioned a MIDI OUT, would it be possible to have both? This would make the shield more versatile imo.