Only one application runs at a time, there is no background processing.

As for Airfoil for the iPhone, at this time, we have no plans for it. Unfortunately, Apple's SDK precludes us from getting any access to audio on the device, and because they're the exclusive way of getting software onto the device, we're stuck playing by their limiting rules. Hopefully, this will change, but currently, we have no way of making Airfoil for the iPhone.