The best way to do this (which is, incidentally, the same method the Zaum website uses) is to create a div that covers the entire visible canvas and fade that OUT once the page fully loads. Here's the code:
#mask {
position:absolute;
top:0;
left:0;
right:0;
bottom:0;
background-color:red;
z-index:99;
}
And the HTML:
<HTML>
<HEAD>
<TITLE>Mask Example</TITLE>
<script type="text/javascript" src="js/jquery-1.3.2.min.js"></script>
<SCRIPT type="text/javascript">
$(window).load(function () {
$('#mask').fadeOut('slow');
});
</SCRIPT>
</HEAD>
<BODY>
<P>Some text goes here</P>
<IMG src="http://spaceflight.nasa.gov/gallery/images/shuttle/sts-119/hires/s119e008352.jpg" />
<P>And moar text!!</P>
<DIV id="mask"></DIV>
</BODY>
</HTML>
Messing with body opacity is generally considered bad practice and may not be fully supported on some browsers.