This is possible, but only with some invalid markup (height is not a supported attribute in the HTML4.01 specifications, so its usefulness is seriously limited):
<table width="100%" height="100%">
<tr>
<td align="center">
<table>
<tr>
<td>Your content goes here</td>
</tr>
</table>
</td>
</tr>
</table>
<table border="0" width="100%" height="100%"
cellpadding="0" cellspacing="0">
<tr>
<td width="100%" height="100%">
Your content goes here
</td>
</tr>
</table>
The above will put a 1 cell table 100% horizontal and vertically
It works with Internet Explorer, Netscape Navigator 4.x and Opera.
Netscape Navigator 6 and Mozilla only support W3C recommendations so those browsers will ignore the "height=100%" attribute and you'll lose vertical centering.