Sample Lab developer, fotoh LLC, have released MagMIDI. A free magnetic Virtual MIDI controller for iPad.
Magnetometer virtual MIDI controller to control pitch wheel and mod wheel of any synth app that supports virtual MIDI. Supports multitasking so you can play keys with one hand in a synth app and have fine control over mod or pitch wheel with a magnet in the other hand.
Using core MIDI, audio backgrounding, along with the Z axis readings off the magnetometer... negative pole vs. positive pole of the magnets will trigger pitch up or down. Sends pitchwheel, modwheel, and random note commands w/ poly-aftertouch.
Made during the 2012 RVA Hackathon: http://rvahackathon.com
What a neat little treat!
They caution against using any extraordinarily strong magnets. I'm not sure about the iPad, most devices with data storage tend not to like strong magnets. Your iPad screen will definitely not be improved by a massive neodymium magnet flinging at it.