I just started the Open Wusik - Midi Projects - area on our site.
Please, check the MIDI Sequencer project. I want to know if people would like this idea, otherwise I may move to a computer based solution and have the arduino only remote control via serial instead. ;-)
I have pretty much all the hardware parts ready in place, just need to start organizing how the code will run and how the midi patterns will be stored in the SD card.