Center Align on a Absolutely Positioned Div

Your problem may be solved if you give your div a fixed width, as follows:

div#thing {
    position: absolute;
    top: 0px;
    z-index: 2;
    width:400px;
    margin-left:-200px;
    left:50%;
}

div#thing
{
    position: absolute;
    width:400px;
    right: 0;
    left: 0;
    margin: auto;
}