Well, I listen mostly to spotify these days. I actually use subsonic mostly because it works great with both desktop and mobile clients, with music that is not on Spotify. It is also of course cheaper, but that is not as important.
So I would love a "plugin" that supports Spotify. I realize this is not likely, seeing subsonic is not really built with this in mind, as far as I can tell any way.
My ideal music solution is for desktop, mobile and "jukebox" clients (remotely control any client) with a server that woks well. This sounds very much like Squeeze Server would do the job, and it _almost_ does. The thing it is lacking is, their spotify plugin requires features that most software players are lacking.