Just last week Audeonic showed us a proof of concept for changing apps with MIDI messages. Today it is available on iTunes!
This allows you to launch an app with a button on your keyboard, or other MIDI controller, or bring apps that are in your background up to the foreground. In order to do that you need a master app telling the other apps when it is their turn to take the stage. That is Audeonic's MIDI Bridge.
Other apps need to be updated now to be OMAC Registry Class compliant in order for MIDI Bridge to tell them to come to the foreground. So far that list only includes other Audeonic apps, but OMAC is something most developers appreciate and support!
For complete details on setting up MIDI Bridge with your own controller, Audeonic has a handy guide, along with a TouchOSC template for experimenting with app switching from iOS itself.
Here is the first demo of Fast-switching with MIDI.