For those times when you don’t want a PC to try to establish a partnership with your Windows Mobile device, you can use this registry file:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows CE Services]
"GuestOnly"=dword:00000001
[HKEY_CURRENT_USER\Software\Microsoft\Windows CE Services]
"Sync window display mode"=dword:00000000

Update 28 July 2009:
As msalvai comments below, it does work with WMDC.