Here is a post in forums.iis.net that is worthy of an complete article.
http://forums.iis.net/t/1150447.aspx
It addresses caching on a UNC path where the server is a non-Microsoft OS file server. It's a Novell server using CIFS. If you have documents on a Samba or CIFS server, this article contains some good information.
Cheers,
Steve