I finally turned off local syncing again and mounted the iDisk in the Finder. I discovered that my Public folder (which looked empty to me) contained over 80 MB of files. But I couldn't see them in the Finder.
Firing up the Terminal, I cded to /Volumes/my_iDisk_name/Public/, and looked inside. Sure enough, when doing ls -al (the -a shows invisible files), I found a handful of temporary files with names beginning with .fstemp. After deleting these files, my space was reclaimed. However, these files must have been generated when a sync was unable to complete. For some reason, the iDisk server didn't erase them. So, if you suddenly lose space on your iDisk, have a look in any folder where you might have tried to sync files to see if there are any such temporary files hanging out.

