Ti faccio un esempio base base:

codice:
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<STYLE type=text/css>

div#box {
 height: 120px; 
 line-height: 120px;
 background-color: #BFC6DE;
 color: #40509B;
 font-family: "Trebuchet MS", Arial, sans-serif
 } 
</STYLE>
</HEAD>
<BODY>

<DIV id=box>Centrato verticalmente</DIV>
height e line-height devono avere lo stesso valore.
</BODY>
</HTML>