place all the content into one div, and have the footer below it
Code:
<div id="head">This is my head</div>
<div id="content">
<div id="rightcolumn">menu item
menu item
menu item</div>
<div id="body">This is my body, blah blah blah</div>
<div id="leftcolumn">This is my left column</div>
</div>
<div id="foot">Yay foot!</div>
The problem with is that your columns may not extend to the bottom of the page. But this is the first solution that came to mind there probaly is a better one.