Custom Walker to format Custom Nav into Columns

I have a custom Nav set-up in my wordpress admin like such:

Custom Nav Screen:
https://img.skitch.com/20120328-fntr9yy5myj3rs9gjmha64y3ry.jpg

I want to display this in my footer in 3 Vertical Columns, like in the below image:

example of output:
https://img.skitch.com/20120328-mpcmanndsgit35a1d648kxk4xp.jpg

What I can’t figure out is how to do a custom walker for wp_nav_menu that will give me something like this html:

<div class="footerCol">
<h3>Top Level Item</h3>
<ul><li>Sub Items [repeat]</li></ul>
</div>
[repeat for each top level in the menu]

Any help would be greatly appreciated….