It's this line that loads rokzoom.js in rt_head_includes.php
<?php if($enable_rokzoom=="true") :?>
<script type="text/javascript" src="<?php echo $this->baseurl; ?>/templates/<?php echo $this->template?>/rokzoom/rokzoom.js"></script>
<?php endif; ?>
If I comment that out the error goes away.
I only gave it a good once over, but I couldn't find anything in rokzoom.js that would be causing this issue.
Content and System plugins of Rokbox are disabled. I'd eventually like to use that functionality - or rokzoom functionality, (as I'm sure you noticed, the site is still being developed), but I don't want my IE8 customers getting an error when they go to my website - looking to buy web services. Not a very good first impression.
Anyway, I'll keep looking it over.