Variable created in hook_preprocess_node is not available to template
The variables added from hook_preprocess_node() are only available in node.tpl.php, or any other node template that Drupal will pick up because any suggestion contained in $variables['theme_hook_suggestions'] (Drupal 7), or $variables['template_files'] (Drupal 6).

To add a variable used by a region template file, you need to use a different preprocess function: hook_preprocess_region(), or hook_preprocess(). The latter is a generic preprocess function; the second parameter passed to that preprocess function tells it which theme hook is going to be preprocessed.

This means that, by default, the region preprocess function doesn't have access to $variables['node']; if you want to access the node object associated with the page being viewed, if there is one, you need to use similar code used in template_preprocess_page() and add the following line to your preprocess function:

if ($node = menu_get_object()) {
  $variables['node'] = $node;
}


The function returns FALSE if the page is not associated with a node; this could happen, for example, if the user is visiting example.com/user/1.