is $node always available in page.tpl.php
$node isn't necessarily defined, from the page.tpl.php Drupal 7 API:


  $node: The node object, if there is an
  automatically-loaded node associated
  with the page, and the node ID is the
  second argument in the page's path
  (e.g. node/12345 and
  node/12345/revisions, but not
  comment/reply/12345).