Код HTML:
<div style="position: relative; width: 100%; height: 100%;">
<div style="position: absolute; top: 50%; margin-top: -10px; height: 20px; width: 100%; text-align: center;">
Some text
</div>
</div>
или
Код HTML:
<html>
<head>
<style type="text/css"><!--
html, body { width: 100%; height: 100%; margin: 0; padding: 0 }
.text { position: relative; width: 100%; height: 100%; }
.text_in { position: absolute; top: 50%; margin-top: -10px; height: 20px; width: 100%; text-align: center; }
--></style>
</head>
<body>
<div class="text">
<div class="text_in">
Some text
</div>
</div>
</body>
</html>