I've set the version to 7.x-1.x-dev but the issue is actually in the 7.x-1.x branch, I can't get -dev to run at all right now. Anyway, the issue here is that variables aren't loaded yet when in authcache_retrieve_cache_page some things don't work as expected. The issue I'm seeing is that Cache-Control's max-age isn't respecting the setting I have. This in turn nullifies the usefulness of varnish for anonymous users.
Inside of include/bootstrap.inc, function drupal_serve_page_from_cache there is a variable_get('page_cache_maximum_age', 0) call being made. Without authcache enabled, this will work properly. But with authcache enabled, Drupal isn't bootstrapped far enough for variables to load. This can be fixed/worked around by adding drupal_bootstrap(DRUPAL_BOOTSTRAP_VARIABLES, FALSE); to authcache.inc in the authcache_retrieve_cache_page function.