I chose to not use MIDI for a few reasons: 1) I wanted to keep the amount of expensive hardware to a minimum; 2) I wanted to be able to use my own samples (while I’m sure some MIDI synths allow this, there is the problem of predefined MIDI notes: for instance, I have 11 distinct samples for the hihat alone (three pizeo sensors, with three possible pedal states each: open, closed, and tight, plus closing sound and splash). I don’t think that MIDI supports this. 3) MIDI supports 7 bit velocity values; I support 10 bits (in practice, this is probably not going to matter, as I am pretty sure the ADC in the Arduino is not accurate enough to matter).
Now, all that being said, it would be quite trivial to modify my plans for use with MIDI, even without changing the board at all. You would need a power adaptor, since you wouldn’t be using USB power; you would also need a 5 pin DIN connector (the MIDI jack). If you use the same board layout, you would need attach the MIDI jack wires to some free ports on the Arduino. The rest is just a matter of software - instead of sending the port:value tuples which I currently send, you would just need to send the MIDI signals (and change the serial pin to be the one you attached the jack to). I have seen many sample programs in these forums and on Google about how to send MIDI data from the Arduino, so you shouldn’t have any problems there.
The hardware design itself is quite agnostic of both input connections and output protocols. With the appropriate jacks, you should even be able to use any standard commercial drum pad instead of building your own. The shield hardware is basically just a number of multiplexers and some filters / rectifiers / amps; nothing fancy at all!