Hey Mike,
I bookmarked your post earlier because I thought there was an easy way to do what you want, but rather than an <iframe /> or a <script /> method, I was looking for a Cascading Style Sheet method. My immediate thought was, {well, why not use "position: fixed;" for the navigation element} but as I looked into it and tested it out, this CSS rule does not work in IE 6.0. It looks really nice in Firefox.

Let me explain a little further. I was going to suggest that you create a <div /> tag with your navigation menu for across the top.
CODE
<div class="topnav"> Home | About | View Cart | Login </div>
You would add this into your top of page editor. Then you would add style information to your 'custom.css' file to arrange it in a fixed position.
CODE
.topnav { position: fixed; left: 0px; top: 0px; }
This would affix it to the location in the browser and as the user scrolled up and down, it would stay in place. The problem is that Internet Explorer 6.0 does not render this properly and reverts to a static position.
It took me a bit of research, but I found a CSS method that will give you an absolute position in IE, and a fixed position in most other browsers. There are other javascript related methods for doing this, but this runs into the same sort of compatibility issues.
For more information and a more detail explanation, a working example, see this link -
http://www.howtocreate.co.uk/fixedPosition.htmlNote: I checked the link that you posted above and it appears that the site did an overhaul to their design and the effect you are looking for is no longer on their site. Perhaps they found out that it wasn't all that browser compatible!

I think the group would appreciate it if you tried this out and were successful with it that you would post again and show it off. I think the affect can be done really well. Thanks!