I recently ran over the following Knowledge Base article published by Microsoft:

SUMMARY
This article describes how to modify your ActiveSync installation to automatically connect devices as “Guest” and avoid creating a new partnership.

MORE INFORMATION
There are times when you might want to establish ActiveSync connections between a Microsoft Windows CE-based device and a desktop computer that is running ActiveSync without creating a new partnership. For these situations, you can configure ActiveSync to automatically connect devices as “Guest” by using the following registry key.

There are times when you might want to establish ActiveSync connections between a Microsoft Windows CE-based device and a desktop computer that is running ActiveSync without creating a new partnership. For these situations, you can configure ActiveSync to automatically connect devices as “Guest” by using the following registry key.

1. Create a text file on the desktop with any name and a “.reg” extension.

2. Enter the lines of text in the file exactly as they appear below and include a blank line at the bottom of the file.

REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows CE Services]
“GuestOnly"=dword:00000001

3. Save the file, and then double-click the file to merge the entries into the registry.

To verify that the values have been entered correctly into the registry, click Start, click Run, type regedit, and then click OK.

The full article can be found here.