Here's an extension which I came across and modified slightly. Looking at it now, I think it's actually a bit messy and could use a clean up (there's some unused variables I think)
$.fn.vCenter = function(options) {
    var pos = {
        sTop : function() {
            return window.pageYOffset || $.boxModel && document.documentElement.scrollTop || document.body.scrollTop;
        },
        wHeight : function() {
            if ($.browser.opera || ($.browser.safari && parseInt($.browser.version) > 520)) {
                return window.innerHeight - (($ (document).height() > window.innerHeight) ? getScrollbarWidth() : 0);
            } else if ($.browser.safari) {
                return window.innerHeight;
            } else {
                return $.boxModel && document.documentElement.clientHeight || document.body.clientHeight;
            }
        }
    };
    return this.each(function(index) {
        if (index == 0) {
            var $this = $(this),
                $w = $(window),
                topPos = ($w.height() - $this.height()) / 2 + $w.scrollTop()
            ;
            if (topPos < 0 && (options == undefined || !options.allowNegative)) topPos = 0;
            $this.css({
                position: 'absolute',
                marginTop: '0',
                top: topPos //pos.sTop() + (pos.wHeight() / 2) - (elHeight / 2)
            });
        }
    });
};
$.fn.hCenter = function(options) {
    return this.each(function(index) {
        if (index == 0) {
            var $this = $(this),
                $d = $(document),
                leftPos = ($d.width() - $this.width()) / 2 + $d.scrollLeft()
            ;
            if (leftPos < 0 && (options == undefined || !options.allowNegative)) leftPos = 0;
            $this.css({
                position: "absolute",
                left: leftPos
            });
        }
    });
};
$.fn.hvCenter = function(options) {
    return this.vCenter(options).hCenter(options);
};
Usage:
$('#verticalCenter').vCenter();
$('#horizontalCenter').hCenter();
$('#bothCentered').hvCenter();