Add a class to a field based on the page regions
A quick look shows that the content region is wrapped in a class derived from that $columns variable, getting it from this function...

/**
 * Returns the correct span class for a region
 */
function _bootstrap_content_span($columns = 1) {
  $class = FALSE;

  switch($columns) {
    case 1:
      $class = 'span12';
      break;
    case 2:
      $class = 'span9';
      break;
    case 3:
      $class = 'span6';
      break;
  }

  return $class;
}


...so you should be able to use the cascading features of CSS to theme your field, eg...

.span12 .your-field-stuff {
  /* widescreen */
}

.span9 .your-field-stuff {
  /* one sidebar */
}

.span6 .your-field-stuff {
  /* two sidebars */
}