NodeBeat is not only an excellent ambient music generator, it is also an excellent ambient MIDI generator. It even supports Virtual MIDI. This is a great app at it's usual price, and even better free!
Here is a demo of it in action from Rheyne.
Details from the video:
NodeBeat's generators are sending MIDI notes to a DSI MoPho, two separate instances of NI's Massive (one polyphonic and one monophonic) and NI's Kontakt. Filters, delays, and reverb in Ableton Live are controlled by Lemur on a 2nd iPad, and Moog's Animoog is running on a third iPad. iKaossilator is running on an iPhone 4, providing some chord stabs. NodeBeat's tempo is set to 60bpm, and iKaossilator is set to 120bpm. Ableton's tempo is also set to 120bpm to keep the delays in sync. Nothing was pre-sequenced or pre-recorded for this jam.