"comment-forum.tpl.php" not discovered by Drupal

I'm using Drupal 6.3.
I had copied comment.tpl.php into my theme's directory, and renamed to comment-forum.tpl.php.

Hardcoded "hello world" to be printed out, appended to the comment's title. This is to prove we are really loading the new template file.

I had searched on Drupal with comment-forum.tpl.php, and there are 2 camps.

First camp, use
function _phptemplate_variables($hook, $vars = array())

> different layout for every page in drupal
> themeing forum comments

Second camp, use comment-forum.tpl.php, and it is highlighted that this is possible in 6.x.
> can we theme comments based on content type?

I belong to the second camp, since I'm using 6.x.
Now the irony is that I still can't get Drupal to discover comment-forum.tpl.php, despite doing any one of these:
i) Administer > Site Configuration > Performance > clear cached data or
ii) Administer > Site Building > Themes > Save Configuration

Can anyone shed some light on this?

Thanks in advance.