Sure enough, my template did not have a call to mootools.js. I added that right before the end of the head and now it's working in IE8.
<script type="text/javascript" src="/templates/MyTemplate/js/mootools.js"></script>
(I also had to copy the JS directory from the milkyway template folder into my template folder.)
So now, I'm wondering, and this will reveal and confirm my total ignorance on this...but why would this work in firefox without the mootools but only in IE with the mootools?
~Neal