From what I see, Subsonic already keeps some information about current sessions, their playlists, etc. A very cool addition would be closer tracking, storage, and retrieval of per-user sessions, including the full playlist and current position in the playlist.
Ideally, I'd be able to create and start streaming a playlist from my web browser, then pause playback, fire up my iPhone/Android app, which logs in under the same username, and pick up right where I left off on my desktop (retrieve my playlist, my position in the playlist, and my position within that song). Similarly, when I close my iPhone/Android client, and come back to the web browser, it would reflect the changes/progress from my portable device's interactions.
I believe the web interface already keeps track of recent sessions' playlists, but not the position in the playlist, and the position in the song. This persistent/portable session feature would definitely be a useful addition.