from: Mike Dimmick's blog
To enable Guest partnership as default when connecting a device via activesync
Modify the registry key to stop the partnership wizard from appearing. Open Registry Editor (as an administrator) and navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows CE Services. Create a new DWORD value called GuestOnly and set its value to 1. The wizard will no longer appear on connecting a device. If you ever do need to create a partnership, you'll need to set this value back to 0 before connecting.