Is there support for '#markup' in field type 'item' in a hook_widget form element?

I'm building a custom field which is display only, it takes data from another field rather than storing any itself.

as part of this, I want to display some information in an appropriate field within the widget, my element in 'hook_widget' therefore looks like:

      $element['data_source_message'] = array(
        '#title'        =>  t('Data display source'),
        '#type'         => "item",
        '#markup'       => "<p>".t('Using the fieldname as the source for the displayed data.')."</p>",
        '#description'  => "<p>".t('Some description about something')."</p>",
      );

everything renders fine, except for the '#markup' element. not sure if this is a bug or if I've missed a trick somewhere.

thoughts anyone?