Thanks to Freesoul for sending me a link to this video.
The Korg iMS-20 is a great synth for the Apple iPad. This video shows how to control this synth with a guitar by using the Sonuus i2M musicport.
The Sonuus i2M musicport converts guitars and other musical instruments to MIDI with very low latency. Being a standard USB MIDI device, it works with the iPad using the iPad camera connection kit.