Edit in JSFiddle

html, body{height:100%; margin:0;padding:0}

.container-fluid{
  height:100%;
  display:table;
  width: 100%;
  padding: 0;
}

.row-fluid {height: 100%; display:table-cell; vertical-align: middle;}

.centering {
  float:none;
  margin:0 auto;
}

<div class="container-fluid">
    <div class="row-fluid" style="background-color: blue">
        <div class="span3 centering text-center" style="background-color: green">
            Yeah I'm centered
        </div>
    </div>
</div>