Allora fai così:

<style type="text/css">
<!--
html {height:101%;}
body {background:#FFCC33;}

#prova1 {
width:100%;
height:auto;
float:left;
background-color:#FFFFFF;
font-size:14px;
border:1px solid #0000FF;}

#wrapper {
width:980px;
margin:0 auto;}

div#prova2{float:left; width:980px; height:150px; margin-top:20px;}
-->
</style>
</head>


<body>
<div id="prova1">
<div id="wrapper">
<div id="prova2">
ciao
</div>
</div>
</div>

</body>
</html>