Overriding theme functions

Hello,

I've been trying yesterday afternoon to create a custom theme function for the theme_item_list(). In my page.tpl.php i use this code :

$output = theme('item_list', $items);
print $output;

If no override is done, Drupal would take its theme_item_list function in /includes/theme.inc
But, according to http://drupal.org/node/171188 all you need to do is add a function in your template.php.

While following this guide, here's what I did:
mytheme.info

; $Id: mytheme.info,v 1.5
name = mytheme
description = mytheme private .
version = VERSION
core = 6.x
engine = phptemplate

template.php

function phptemplate_item_list ($items = array(), $title = NULL, $type = 'ul', $attributes = NULL) {
  return $output.'<h1>testing?</h1>'; 
}

page.tpl.php

$output = theme('item_list', $items);
print $output;

As you would've probably guessed... this didn't change anything. Drupal just doesn't take or execute this function.
Is there something I missed?

Thanks for you help,
cheers,
micobros