In response to @smashingmag's tweet: https://twitter.com/smashingmag/status/306416073408929792 an alternative method.

nbspBetweenLastWords.php

<?php

function nbspBetweenLastWords( $text, $minWords = 3 ) {
  if (substr_count($text, ' ') >= $minWords - 1) {
    return substr_replace($text, '&nbsp;', strrpos($text, ' '), 1);
  }
  return $text;
}