From the Android developer guide (
http://developer.android.com):
If a user unmounts their phone's SD card (for external drive use, etc), certain consequences apply. One is:
"Your App Widget will be removed from the home screen. When external storage is remounted, your App Widget will not be available for the user to select until the system resets the home application (usually not until a system reboot)."
So, it sounds like if you move Subsonic to your SD card and mount your phone as external storage on your PC, the Subsonic widget may go away.