Overriding theme form functions

Hi,
I want to override field.module's theme_field_multiple_value_form() which themes the multiple value form. I've done it in D6, but I seem to be missing something because my override function does not get called.

the function call in "field.form.inc":

function theme_field_multiple_value_form($variables) {

my hook_theme declaration:

function mymodule_theme ($existing, $type, $theme, $path) {
 return array(
    'field_multiple_value_form' => array('render element' => 'element'),
    'mymodule_field_multiple_value_form' => array('render element' => 'element'),
  );

my function call

function mymodule_field_multiple_value_form($vars) {

Thanks!