Preprocess function not doing what it should

Hi,

I'm using Footheme as a subtheme of Sky & generally, everything works great.

I'm trying to use a theme_preprocess_node() function to change the submitted text from the default set by AT. This is the function:

function mytheme_preprocess_node(&$vars) {
  if (variable_get('node_submitted_' . $vars['node']->type, TRUE)) {
    $vars['submitted'] = t('By !username on !datetime',
      array(
        '!username' => $vars['name'],
        '!datetime' => date('j/n/y',$vars['changed']),
      )
    );
  }
}

When I include this snippet in Sky's template.php, the submitted text changes. However, when it's in Sky's subtheme, it does not change. I've done all the cache clears I should and I can confirm that a dsm($vars); before the function closes (in the Sky subtheme) does suggest that $vars['submitted'] has changed. However, the submitted text that appears is the AT default.

Please tell me I'm not going mad!!

Matthew