Whenever MBL’s firmware updates, the MBL setting that keeps (all or chosen type of) media shares private gets reset, and in fact set to the default, which is “ALL”. Therefore files which the User wishes to keep private are now available to all on the network. Anyone on the network accessing Twonky, for example, will now see all media files from all users.
Screenshot of my MBL’s UI immediately after a firmware update:
(note that when the ‘Media Serving’ dropdown is set to ‘None’, the icons on the left in the User area are greyed-out)
FYI: this issue is not new; I noticed it not long after I got my MBL in the summer of 2011, and have had to manually reset my Media Serving preference every time there is a new Firmware update.