Position.center = function(element) {
    var options = Object.extend({ zIndex: 999, update: false }, arguments[1] || {});
    element = $(element);
    
    if(!element._centered) {
        Element.setStyle(element, {position: 'absolute', zIndex: options.zIndex });
        element._centered = true;
    }
    
    var dims = Element.getDimensions(element);
    
    Position.prepare();
    
    var winWidth = self.innerWidth || document.documentElement.clientWidth || document.body.clientWidth || 0;
    var winHeight = self.innerHeight || document.documentElement.clientHeight || document.body.clientHeight || 0;
    var offLeft = (Position.deltaX + Math.floor((winWidth-dims.width)/2));
    var offTop = (Position.deltaY + Math.floor((winHeight-dims.height)/2));
    
    element.style.top = ((offTop != null && offTop > 0) ? offTop : '0') + 'px';
    element.style.left = ((offLeft != null && offLeft > 0) ? offLeft : '0') + 'px';
    
    if(options.update) {
        Event.observe(window, 'resize', function(evt) {
            Position.center(element);
        }, false);
        
        Event.observe(window, 'scroll', function(evt) {
            Position.center(element);
        }, false);
    }
}