var my_width = 0;
var my_height = 0;

if ( typeof( window.innerWidth ) == 'number' )
{

my_width = window.innerWidth;
my_height = window.innerHeight;
}
else if ( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) )
{

my_width = document.documentElement.clientWidth;
my_height = document.documentElement.clientHeight;
}
else if ( document.body && ( document.body.clientWidth || document.body.clientHeight ) )
{

my_width = document.body.clientWidth;
my_height = document.body.clientHeight;
}

element.style.position = 'absolute';
element.style.display = 'block';
element.style.zIndex = 99;

var scrollY = 0;

if ( document.documentElement && document.documentElement.scrollTop )
{
scrollY = document.documentElement.scrollTop;
}
else if ( document.body && document.body.scrollTop )
{
scrollY = document.body.scrollTop;
}
else if ( window.pageYOffset )
{
scrollY = window.pageYOffset;
}
else if ( window.scrollY )
{
scrollY = window.scrollY;
}

var elementDimensions = Element.getDimensions(element);

var setX = ( my_width - elementDimensions.width ) / 2;
var setY = ( my_height - elementDimensions.height ) / 2 + scrollY;

setX = ( setX

This will center your div if you do something like this on Javascript to the div in question:


Effect.Center('idOfMyDiv')


Try it, it works.
You might want to hack the function in case you don't want this effect to center vertically.

Also, if you want to keep centering the div even on window resize or scroll, you'll have to add
callbacks to window.onresize and window.onscroll

And remember, Linux Rules.