I'm no longer sure that "GuestOnly" is really the registry key I need to change to prevent file syncing between a handheld device and the PC to which it is connected.
If I open ActiveStync's "Windows Mobile Device Center", select "Mobile Device Settings" then "Change content sync settings" and check or uncheck "Files - Sync Settings", it does not alter the value of the "GuestOnly" registry key in "SOFTWARE\Microsoft\Windows CE Services"
The "GuestOnly" value remains 0x00000000 (0) no matter whether I check or uncheck "Files - Sync Settings" (I do mash F5 to refresh the display in regedit).
So is there some other registry key that corresponds to toggling "Files - Sync Settings", or have all my travails to set this value been to no avail? There must be some registry key that's getting set...