Document Start Test Script available here http://userscripts.org/scripts/show/78045

Document Start Test .user.js

// ==UserScript==
// @name           Document Start Test
// @namespace      vidzbigger.com
// @description    This scrip tests document-start
// @include        http://*
// @run-at         document-start
// ==/UserScript==

GM_addStyle('body{display:none;}');

var isDocumentStart = (document.body==null);

function onLoaded(){
	document.body.appendChild(document.createTextNode('Code that executes at Document Load'));
	GM_addStyle('body{display:block;}');
}

function onDomNodeInserted(n){
	console.log(n.relatedNode);
}

if( isDocumentStart ){
	alert(' Hello!  You are now running at document start! ');
	document.addEventListener('DOMNodeInserted',onDomNodeInserted,true);
}else{
	alert(' Hello!  Script is running at domContentLoaded ' );
}

document.addEventListener('DOMContentLoaded',onLoaded,true);

Note that the old b89971 revision is BAD because it simply calls the onDomContentLoaded() function when not running at document start - this is bad because it will break fastback functionality since it will end up getting called twice.