A while back, I saw this post here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1197235191, and I was so impressed and inspired, that I decided that I'd make my own! My version isn't quite as polished, but it is pretty close. I have added MIDI IN on my version, but it has been tremendously difficult to get that to work in software thus far, even with the MIDI libraries some have made!
I took a touchpad from an old Dell Inspiron 1100 laptop, traced the PCB connections, and implemented it with a surprisingly little amount of trouble. I added a single potentiometer, a tri-color ('RGB') LED, DPDT switch to add ease of programming (switches serial connections so MIDI interface does not interfere with FT232 USB->Serial programming/debugging interface).
It also has a MIDI IN and OUT jacks, and a large rocker power switch with a simple but very effective power circuit that uses a transistor NOT gate (made with a single BC548B NPN transistor), and a TIP120 power transistor among a few other components. This circuit is necessary to protect the 5V USB and regulated 5V voltages from doubling up and damaging things if the system is connected to a USB port and external power source (i.e. wall tansformer, battery pack) at the same time.
I attempted this a while back, but I made it so complex that it became impossible to debug and develop any further, and I abandoned it. So I took a different approach this time and kept things simple (sortof), and I've made all the parts modular so it is easier to work with.
I have yet to make some more I/O functionality; I plan to add multiple (probably 8 or 16) potentiometers with accompanying LEDs and pushbuttons for each, to make it a MIDI sequencer.
Here are some pictures:
