Just came back to computer after 4-5 hours to find my entire documents folder deleted....... I hadn't tried to move anything to the trash..... just left the thing running.... came back to nothing! Any ideas anyone? 
Can you back up your iDisk contents to through Time Machine?
If you have "local iDisk" turned ON then the contents of your iDisk is saved on your Mac as a "Sparse Disk Image Bundle".
Yes it is. When you "Start" or turn on local iDisk you are creating a local copy of your iDisk on your computer for use when you do not have an internet connection.Is "local iDisk" turned ON the same as "iDisk Sync" turned ON (Update: Automatically) under MobileMe in System Pref?