MobileMe, iDisk: About the invisible ".filler.idsff" file
Summary
If you have enabled iDisk Syncing and look at your iDisk's directory in Terminal or another application that allows you to see invisible files, you may notice a file named ".filler.idsff". This is normal.
Products Affected
Mac OS X 10.4, Mac OS X 10.5, MobileMe iDisk, MobileMe
This file is used as part of iDisk Syncing to determine the amount of free space on your iDisk. The size of this file may vary.
Deleting the file is not recommended; doing so does not recover any iDisk or hard drive space. If you have deleted the file, you should restart your computer to recreate the file.