Vertically & Horizontally Centered <div>

Ran across this beautiful CSS example of a perfectly centered <div>:

#mydiv
{
    position:absolute;  
    top: 50%;  
    left: 50%;  
    width:30em;  
    height:18em;  
    margin-top: -9em; /*set to a negative number 1/2 of your height*/  
    margin-left: -15em; /*set to a negative number 1/2 of your width*/
}