ehm, beh, l'effetto che scrolla con la pagina si ottiene con i CSS, con JS si setta soltanto il margin-top ed il pulsante per impostare il display: none;

Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
        <title>Untitled Document</title>
        <style type="text/css">
            *{
                margin: 0;
                padding: 0;
            }
            #Bottomed{
                height: 150px;
                width: 100%;
                position: fixed;    
                background-color: red;
                margin: 0;        
            }
            #Bottomed div{
                text-align: right;
                height: 20px;
                line-height: 20px;
            }
        </style>
        <script type="text/javascript">
            window.onload = function(){
                var myHeight = 0;
                if( typeof( window.innerWidth ) == 'number' ) {
                    //Non-IE
                    myHeight = window.innerHeight;
                } else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
                    //IE 6+ in 'standards compliant mode'
                    myHeight = document.documentElement.clientHeight;
                } else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
                    //IE 4 compatible
                    myHeight = document.body.clientHeight;
                }                
                var btm = document.getElementById('Bottomed');
                var height = window.innerHeight ? window.innerHeight : document.documentElement.clientHeight;
                btm.style.marginTop = (myHeight - 150)+"px";
                
                var closing = document.getElementById('Closing');
                closing.onclick = function(){
                    document.getElementById('Bottomed').style.display = 'none';
                }
            }            
        </script>
    </head>
    <body>
        <div id="Bottomed">
            <div>
                <a href="#" id="Closing">
                    Close X
                </a>
            </div>
        </div>
    </body>
</html>