Center span in Twitter Bootstrap

HTML

<script src="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/2.1.0/bootstrap.min.js"></script>
<div class="container">
  <div class="span6 centred">
      I am a centred div.
  </div>
</div>

CSS

@import url('http://twitter.github.com/bootstrap/assets/css/bootstrap.css');
[class*="span"].centred {
  margin-left: auto;
  margin-right: auto;
  float: none;
  background-color: #ccc;
}