Something

$vocabularies = variable_get('taxo_drill_down_vocabularies', array());

$terms = taxonomy_get_children($tid,$vid);

$options = array();
foreach($terms as $term) {
$options = array_merge($options,array($term->tid => $term->name));
}

$form['Taxonomy'] = array(
'#type' => 'select',
'#multiple' => TRUE,
'#size' => 10,
'#title' => t('Categories'),
'#options' => $options,
//'#tree' => TRUE,
);

// To style the form title: 
// #prefix => div class="foo" 
// .foo label { ... }  
// this would wrap the entire form-item class div
// and hence the label    


$form['submit'] = array(
'#type' => 'submit',
'#value' => t('GO!'),
);

return drupal_get_form('taxo_drill_down_page', $form);