[AndrewGSW]

Maybe

<iframe id="content" name="content" src="content.asp" width="80%" frameborder="0" marginheight="0" marginwidth="0"></iframe>
<script type="text/javascript">
function resizeIframe() {
var height = document.documentElement.clientHeight;
if ( this.offsetTop )
    height -= this.offsetTop;

// not sure how to get this dynamically
height -= 20; /* whatever you set your body bottom margin/padding to be */

this.style.height = height;

};
document.getElementById('content').onload = resizeIframe;
document.getElementById('left_page').onload = resizeIframe;
document.getElementById('right_page').onload = resizeIframe;
window.onresize = resizeIframe;

this should refer to the element that triggered the event, I believe.