A me non si allinea nel box. Allora il problema deve essere a livello superiore.
Il corpo è composto da tre colonne. In quella sx ci sono due box uguali in verticale, in quella dx idem solo che il box superiore deve contenere i dati di login.
Posto l'intero codice:

<body><center>
<div id="main"><center>
<?php require_once("__testata.php")?>
<div id="corpo">
<div id="corpo_sx">
<div id="box_sx">
</div>
<div id="box_sx">
</div>
</div>


<div id="corpo_centro">
Bla... bla... bla....
</div>


<div id="corpo_dx">

<div id="box_login">
<form id="form" method="post" action="">
<fieldset>
<div>
<label for="key">Utente </label>
<input name="key" type="text" id="key" value="<?php echo $key?>" />
</div>
<div>
<label for="pwd">Password </label>
<input name="pwd" type="password" id="pwd" value="<?php echo $pwd?>" />
</div>
<div class="form-button">
<input type="submit" id="go" value="login">
</div>
</fieldset>
</form>
</div>


<div id="box_sx">
</div>

</div>

</div>

<?php require_once("__piede.php")?>
</center></div>
</center></body>
</html>




body {
background-color: #ffffff;
margin: 0px;
padding: 0px;
font-family: "Trebuchet MS", Arial, Verdana;
font-size: 85%;
color: #333333;
}

#box_login {
margin: 0px;
margin-top: 20px;
padding: 0px;
width: 90%;
height: 150px;
border:solid 1px #dddddd}

#form{ width: 100%;
min-width:150px}

#form fieldset{
border:0px solid;
margin:0;
padding:0}

#form div{padding: 10px 0;
color: #333;
border-top: 0px solid #CCC}

#form label{float: left;
width: 33%;
line-height: 23px;
padding-right: 2%;
text-align:right}

#form fieldset input,textarea{width:58%}

#form div.form-button input{margin-left:35%}


#main {
background-color: #FFFFFF;
width: 100%;
padding: 0px;
margin: 0px;
text-align: left;
}

#corpo {
width: 100%;
margin-top: 0px;
margin-right: 0px;
margin-bottom: 0px;
margin-left: 0px;
padding-bottom: 0px;
height: 380px;
margin: 0 auto;
}


#corpo #corpo_sx {

width: 24%;
height: 350px;
margin: 0px;
padding: 0px;
float:left;
}

#corpo #corpo_centro {
background-color:#eaeaea;
width: 50%;
height: 350px;
margin: 0px;
margin-left: 10px;
padding: 0px;
float:left;
}

#corpo #corpo_dx {
width: 24%;
height: 350px;
margin: 0px;
padding: 0px;
float:right;
}

#box_sx {
margin: 0px;
margin-top: 20px;
padding: 0px;
width: 90%;
height: 150px;
border:solid 1px #dddddd;
}