Why would $vars['conditional_styles'] be empty?

In my .info file:

stylesheets[screen][] = styles/reset.css
stylesheets[screen][] = styles/960.css
stylesheets[screen][] = styles/core.css
conditional-stylesheets[IE][screen][] = styles/ie.css

In my template.php file:

function mytheme_preprocess_page(&$vars) {

  if ($vars['is_front']) {
    $vars['css']['screen']['theme']['sites/all/themes/mytheme/styles/homepage.css'] = 1;
    $vars['styles'] = drupal_get_css($vars['css']);
  }
  $vars['styles'] .= $vars['conditional_styles'] = variable_get('conditional_styles_' . $GLOBALS['theme'], '');

}

The "conditional_styles" key exists in the $vars array, but $vars['conditional_styles'] is empty. I have cleared the cache. Have I missed something?