The many layers of caching Drupal. http://drupalwatchdog.com/1/1/many-layers-caching-drupal
Pressflow, Varnisch and caching. http://blogs.osuosl.org/gchaix/2009/10/12/pressflow-varnish-and-caching/
If you've enabled voting for anonymous users. You should disable the page cache for pages containing a poll node.
