This is ugly as hell, but works on Sony 505 and ADE/Windows 1.7.2. No SVG, so it should (does it?) work on every ePub reader.
Code:
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>ePub Vertical Centering - Test Page</title>
<style type="text/css">
html, body {height:100%; margin:0; padding:0;}
.wedge
{float:left; height:50%; margin-bottom:-2em;}
.container
{clear:both; height:4em; position:relative;}
table, tr, th {height:4em;width:100%;text-align:center;}
</style>
</head>
<body>
<div class="wedge" />
<div class="container">
<table><tr><td>
<p>Bull's eye!</p>
</td></tr></table>
</div>
</body>
</html>