Just in case you haven’t heard, Audiobus has been approved by Apple! This is definitely a milestone / landmark for iOS audio. No more copying from app a to app b, or using Virtual MIDI to get multiple apps playing at the same time and sacrificing the mix. In addition to opening up new mixing possibilities, this will just make things easier by letting you hook apps up to each other and access record/play buttons (and hopefully more settings) from any Audiobus supported app with the in-app connection panel. But most importantly, Audiobus will allow you to play audio in app a and process it live in app b. Stay tuned everybody, this is going to be epic! iOS music making will never be the same.
See Also: Synthtopia Audiobus interview