This Drupal 5 version of the varnish.module requires Pressflow for Drupal5. More information about setting Varnish up is available at:


Project Mercury Documentation
http://groups.drupal.org/pantheon/documentation

Varnish and Drupal
http://varnish-cache.org/wiki/VarnishAndDrupal

Example Varnish VCL for a Drupal / Pressflow site
http://highervisibilitywebsites.com/example-varnish-vcl-drupal-pressflow-site