Autocomplete Drupal Form Tutorial.

I got a chance to use drupal form '#autocomplete_path' attribute to my one of the project.

Example usage :
// Autocomplete field

$form['autocomplete'] = array(

 '#type' => 'textfield',

 '#title' => 'Autocomplete field',

 '#description' => 'Autocomplete field example',

 '#autocomplete_path' => 'article/autocomplete',

  );

I have found one good resource about it at http://www.coalmarch.com/blog/autocomplete-drupal-form-tutorial

It may be useful to you. Happy coding :)