It would be awesome if it displayed the album cover of the music being streamed to each zone, and allowed you 'link' the volume control so all zones could be controlled at once, or unlinked for independent control. To me, an avid Subsonic user both at home and remotely with my Android, it would be ideal having Subsonic (via iPod Touch) controlling all the audio throughout my whole system.
I'm not sure how easy it would be to allow Subsonic to address a sound card with these outputs, but I figure if it can be configured to send audio to various outputs of our choosing, then it's kinda' platform/hardware independent and opens up a whole new market for the Subsonic software.
Example of an Airplay app:
Example of a multizone audio card:
