Alter Registration Form Text

I am trying to alter the email description text on my user registration page. I have added the following to the bottom of my template.php file but nothing is happening. It seems like the function isn't even being called. Any suggestions?

function theme_form_alter(&$form, &$form_state, $form_id){    switch($form_id) {        case 'user_register_form':            $form['account']['mail']['#description'] = t('E-mail Address');    }}