Actually - Patrick's code sort of worked for me as well. The correct way to do it would be along the lines of this:
Note: there's a bit of jquery ahead:
if ($.browser.msie == false) {
var h = (document.getElementById("iframeID").contentDocument.body.offsetHeight);
} else {
var h = (document.getElementById("iframeID").Document.body.scrollHeight);
}