Not sure if this already has been brought up, but I've recently looked at Songbird, an open source music player with imho a lot of potential.
Now, include Subsonic and that potential is huge! I can't express how sweet it would be to see Subsonic as a potential source.
There's a few extensions already to include DAAP servers etc, might be possible to look at that source to see what's required.
Or alternatively, make subsonic behave like a DAAP server (instead of inventing a new server/client-scheme) as that would allow players like iTunes to connect by itself.
If this is already possible, please excuse me as I haven't upgraded my subsonic installation yet.