Here’s a slightly bizarro take on controlling your music apps.
Alkex has released Magic MIDI, a free app that lets you use an iPhone as a 16-pad MIDI controller – no special MIDI cables required.
Instead, Magic MIDI uses audio to transmit the MIDI signals to your computer, over a standard male-to-male minijack cable. A free app (Mac, PC or Linux) is required on your PC, which turns the audio back to MIDI signals your apps can handle.
See the Alkex site for details.
Note: As of this post, Magic MIDI is not available in the US App Store.