IK Multimedia has designed a MIDI interface for the iPhone, iPod touch and iPad. iRig MIDI works with apps like Garageband that support Core MIDI. The interface comes with a MIDI recorder and SampleTank Free, a rompler with a sound library of 500 instruments. The sound library takes 1 gigabyte of space.
iRig MIDI costs € 55 ($ 70). The video below shows the features of iRig MIDI: