Does the iPhone need a mobile MIDI app? Irrelevant question, self, because it has one now, regardless of need or want. Called the Line 6 MIDI Mobilizer, it's a low power, portable option for MIDI mixers on the move.
The free app portion is available now, but you'll have to wait until the hardware goes on sale sometime in "Spring 2010" to use it in any meaningful kind of way. But once the entire kit is available, oh the MIDI recording, storage and transferring to computers you will do! [Line 6 via Engadget]