Add a class to all title text fields in all content add forms
The content title is a property rather than a field, so overriding theme_field() isn't the way to do this.

Instead you might want to use a hook_form_FORM_ID_alter() (which can also be used in your theme's template.php file):

function ccom_form_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['title']['#attributes']['class'][] = 'span6';
}