I made a project that includes Arduino in it. It is a MIDI controller with footboard. In current implementation Arduino is used only for getting the status of keys and knobs and sending them via serial. More interesting part is computer software. It is a Python script which creates a GUI that makes possible to bind any number of MIDI messages to any control. It works on Linux. Future plans include adding real MIDI support to device so it can be connected directly to MIDI equipment and edited onboard.
More info here: http://hyperglitch.com/articles/ctrlr