Theming custom module

Hi,

I'm developing a custom module. I have found this useful article about overriding theme functions. http://drupal.org/node/11811. It's working correctly for theme functions already in the drupal theme system. For example: theme_item_list();

The problem is: I have wrote my own code, but it seems drupal does not recognize it like a node theme:

function phptemplate_esorg_item($items = array(), $title = NULL) {
  return _phptemplate_callback('esorg_item', array('items' => $items, 'title' => $title));
}

I've wrote my template file: esorg_item.tpl.php

<div class="node<?php print ($sticky) ? " sticky" : ""; ?>">
  <? foreach ($items as $item) {?>
    <? print $item?>  
  <? } ?>
</div>

...and from my custom module I call the theme like this:

  $items = Array("Test1", "Test2", "Test3");
  print theme("esorg_item", $items, "Test page");

The output I get, when I navigate to my module in a webbrowser is:

  <div class="node">
  Test1  Test2  Test3
      <h2><a href="" title="Test page">Test page</a></h2>
    <div class="node-content">
     </div>
    </div>

So as you see, there is missing the full page template html code. It seems drupal recognize my module as a page template, like: page.tpl.php, but I would like to use my theme function output as a node.

If I modify the theme_item_list(); (or any other that is already in drupal) functions, like in the article, it is works correctly, but as I change the name of the theme function it is not.

Any idea? I think I have to tell drupal about my theme function that is not a page theme function, but I can't figure out how?

Thanx
Tamas