In response to your comment about MIDI: while the version I designed does not use MIDI (for a number of reasons; see FAQ http://drummaster.thecave.homeunix.org/faq.jsp for more details), it would be very possible to modify the design slightly, mostly in software, to allow MIDI to work.

To use MIDI, you would need to do the following:
1) Get a MIDI jack (5 pin DIN IIRC), and hook up the correct wires. You would have to hook directly to the Arduino, instead of the shield, but since my shield design only connects to the pins which are needed, this is not a problem.
2) Modify the Arduino software to send a MIDI note instead of the (port:value) tuple. You will probably have to bit shift the velocity value 3 bits, as my protocol specifies 10bit velocities, and MIDI uses 7.
3) Get a power adaptor for your Arduino, as you can’t use USB power anymore

There are a number of pages which do something similar to this, notably http://todbot.com/blog/2006/10/29/spooky-arduino-projects-4-and-musical-arduino/ . This site, while self-professed by the author as being more of a toy than a real drum brain, has MIDI details on both the hardware and software side.

Cheers