Well I had vague recollections of trying to get this to work last time and I believe it all cocked up because the installer removes the old service and creates a new and I didn't take a note of how it was running before the install.
But I did get it to work. I have a user account with the same login/password on the MP3 server and on machine running Subsonic (can't run Subsonic on the MP3 server because it's hard-coded in the installer and somewhere inside the program to use the C: drive as the root drive. Install Windows to anything but C: and Subsonic fails to run (or fully install)).
The user account can access the shared MP3 folder without issue from the Subsonic machine. Subsonic is set in the Services applet to login as the user (and not Local System Account).
Then in Subsonic, when you go to add the folder to the library, don't try using a drive letter mapping (because the service may not see it) but instead use the UNC path. So while my user on the Subsonic computer can 'see' M:\Music, I set Subsonic up to use \\mp3server\m$\media\music instead (you may need to use / instead of \ depending on your setup).
That worked for me, I'm now back to listening music at work.
EDIT: Oh yeah, this was reinstalling using 4.0.1. I'll try the same stunt again when I'm at home to see if it works on 4.1.