question about caching

I have in environment.rb:
config.action_controller.page_cache_directory = RAILS_ROOT + "/public/

The following rules in lighttpd.conf:
url.rewrite += ( "^/$" => "/cache/index.html" )
url.rewrite += ( "^([^.]+)$" => "/cache/$1.html" )

What I'm seeing is that if a page has already been cached (and exists
in public/cache), it gets served properly. However, if the page hasn't
yet been cached, mongrel is looking for the cached page, can't find it
and throws a Routing Error.

I followed this article pretty closely:

Any ideas?