XML File

You could *fake* it through mod_rewrite and .htaccess.

http://boren.nu/archives/2005/03/29/feed-director-plugin/

Beyond that you’d need a way to output rss content to a static file. Something like this could be scripted, but you’d lose a lot of the flexibility WordPress’ dymanic method provides (i.e. not only would you need to rewrite it each time you update, but you’d need a file for every category, every archive type, each author, each post comment thread, etc.).