You know what might be a decent solution for this? Adobe AIR.
SitePen, Inc. has created a program built on AIR called
Queued to use with Netflix. It essentially takes the Netflix data and makes it beautiful. It also has a lot of offline features and it acts like a normal program.
You could always use
Mozilla Prism to make a semi-standalone app, but I'm pretty sure you can't have it sit in the tray and have controls with it, like for iTunes or something.
Anyways, maybe if someone could develop a program built on AIR that connects to the Subsonic server, this would make a lot of people happy. It's just an idea.