var elt         = $('ID_DIV_TO_CENTER');

// retrieve required dimensions
var eltDims     = elt.getDimensions();
var browserDims = document.body.getDimensions();

// calculate the center of the page using the browser and element dimensions
var y  = (browserDims.height - eltDims.height) / 2;
var x = (browserDims.width - eltDims.width) / 2;

// set the style of the element so it is centered
var styles = { position : 'absolute',
	top      : y + 'px',
	left     : x + 'px' };

elt.setStyle(styles);
elt.scrollTo();

http://www.barattalo.it/

javascript to center a div with prototype.js

javascript, center

JavaScript