[Lynne]

Just write a plugin like:
hook location - search_complete

PHP Code:

$this->template->register('criteria', $_GET['tag']); 


And then use the variable criteria in the <title> tag.

Probably should clean it first. So...

PHP Code:

$vbulletin->input->clean_array_gpc('r', array(
        'tag'    => TYPE_STR,
));
$this->template->register('criteria', $vbulletin->GPC['tag']);