If the page is dynamic then, point root to the right controller and
action.
map.root :controller => 'yourcontroller', :action => 'youraction'
If it's static then you'll better think twice if you need it, and if
you realy need then try one of the following things:
1. Symlink it to the /index.html
2. Try Apache DirectoryIndex, or nginx index directives, although I'm
not sure that they allow setting the default page in another
directory.
3. Use URL rewriting on the webserver level