Hey Donncha,
I think I had the accepted filenames correct last week when I was having this challenge. Unfortunately I don't remember now, but after several days of frustration, I decided to wipe the new server install. Also, I had some other configuration that I wanted in place on the server, so it was not just because of this caching issue (or at least I thought it was cache related).
Now I've got a linux server with Nginx serving out static files and spawn-fastcgi taking care of WPMU's php concerns. I also was able to successfully configure your WP-Super-cache to work with this setup, and have the cached html files served out with nginx.
All in all pretty sleek set up and I'm not having any challenges with the admin pages being cached. As a bonus, even the admin pages are serving to the browser more quickly than any Apache set up I've had in the past. Speed in administering the site is always groovy.
Thanks for your reply. Much appreciated!