body {
margin:0;
padding:0;
background-image:url(/images/bg_enter.jpg);
text-align:center; /* hack para o IE */
}

#divCentralizada {  
            position: absolute;  
            width: 600px;  
            height: 300px;  
            left: 50%;  
            top:50%;  
            margin-left: -300px;  
            margin-top: -150px;  
         }