I've tried this on a completely fresh install of 2.2RC2a, but it doesn't seem to work - I get an error :-
Builder is not defined
http://webtest/osc/lightbox/lightbox.js
Line 132
objBody.appendChild(Builder.node('div',{id:'overlay'}));
I've tried amending product_info.php and adding builder to this line:
<script type="text/javascript" src="lightbox/scriptaculous.js?load=effects,builder"></script> .
but this doesn't seem to fix the problem - I don't get the error anymore, but (in IE) don't get the original smaller image. Firefox displays the image, but nothing happens when I click on it.