WebDAV : how to serve files from other servers

I have installed WebDAV in my webserver with windows authentication enabled
so users can connect to it using their username and password.

As a test, I set up a virtual folder under IIS which points to a folder on
the webserver to act as a WebDAV share : this worked fine and users can
navigate the folder content over the internet.

However...here's the problem...

The files I actual need to share are not located on the webserver, they are
located on other servers. I use DFS to bring all of the various shared
folders together under a single root so that it's easy for the users to
understand and use them.

I want the users to see the same structure when using WebDAV as they do when
on the LAN.

So, how do I configure the virtual folder under IIS to point to the DFS root
in such a way that external users can use WebDAV to access the shares ?

Update...

The following white paper describes how to achieve what we're after:

http://www.microsoft.com/technet/prodtechnol/windowsserver20 03/technologies/webapp/iis/remstorg.mspx

The only thing that we can't get working is having users automatically
connect to the WebDAV folder using their logged on credentials. By this I
mean that a user would log on to Windows on a remote PC, connect to the
internet and then *not* be prompted for credentials when connecting to the
WebDAV share. Currently, users have to enter their username and password
everytime they connect to the WebDAV share - it would be good if their logged
on credentials could be used to authenticate with IIS in the background to
provide a "seamless experience".

Any ides ?

Edward