Brilliant, thanks!
With some slight modifications it not only works for text but also for images!
Code:
<html>
<head>
<style type="text/css">
html, body {height:100%; margin:0; padding:0;}
.wedge {float:left; height:50%; margin-bottom: %image-height/2%;}
.container {clear:both; height:0em; position: relative;}
table, tr, th {height: %image-height%; width:100%; text-align:center;}
</style>
</head>
<body>
<div class="wedge" />
<div class="container">
<table><tr><td>
<img src="%image%" />
</td></tr></table>
</div>
</body>
</html>