Adding some of my own research results to the excellent findings allready posted by Ace.
1) The stereo guys do have a DACP server implementation
http://code.google.com/p/stereo/
They actualy use the iphone remote app as their reference client though still with a jailbreaked phone (since the pairing wasn’t figured out at the time..which is now..see Ace’s link [2])
2) For using an airport express a number of RAOP implementation are out there (though still hard to find)
– Pulseaudio has RAOP output (which would be a good independent and flexible way if someone wants to implement a full solution)
– VLC speaks RAOP though hardly anyone ever heard about that one 😉
– there is a ruby based implementation which is working fine and great for rapid prototyping something
So basicaly every needed component is there. It is just about to get this together and nicely package it. Agree that this is not to go into firefly but rather a good extension to it kept in a seperate package.
In the mean time I’ve created a “similar” setup using mpd with the iphone mpod app on my NAS. Almost but not quite the real iThing.. (google “mpod raop mpd” and follow first result if you like more info)
I’m lacking some to really get this all together in a useable form. But if some is keen addressing this I’m willing to help.