Silicon Studios has launched a number of iPhone apps that tie into their small desktop stub to provide MIDI control from the iPhone. Basically, you run an app on your iPhone, which connects to your desktop over wifi, which creates a MIDI interface on your computer and pipes in control from the phone.
So, get the desktop app from iTouchMidi.com, get the free demo app from the iTunes store, and start playing. Launch Motion, add a filter to a layer, then right click on any filter control and select “midi” – now you can control that setting from the phone. This is highly entertaining.
Confused? This video might help (large one here):