tpl.php files in subfolders
In D6, any custom templates (from core, such as node or page) must be in the same folder as the base tpl file in order to use the auto-recognition of theme files. 

You may be able to specify additional template files (in D7 they're called theme hook suggestions) in a preprocess function in your theme's template.php. See the first code example at http://drupal.org/node/249726 to get you started.