How to highlight active teaser in Views?
The following does the work for me:

function YOUR_THEME_preprocess_views_view_unformatted(&amp;$vars) {

  if (arg(0) != 'node') return; // no active node

  $index = 0;
  foreach($vars['view']-&gt;result as $node) {
    if (arg(1) == $node-&gt;nid) {
      $vars['classes_array'][$index] .= ' active';
    }
    $index++;
  }

}