SOHideSystemMessage.user.js

// ==UserScript==
// @name         SO Hide system message
// @namespace    http://github.com/Tiny-Giant
// @version      0.1
// @description  Hide system messages on Stack Overflow
// @author       @TinyGiant
// @match        http://*.stackoverflow.com/*
// @grant        none
// @run-at       document-start
// ==/UserScript==

(function(){
    "use strict";
    if(!localStorage['SOHideSystemMessage_done']) {
        var m, interval = setInterval(function(){
            if(localStorage['SOHideSystemMessage_done']) clearInterval(interval);
            m = document.getElementById('system-message')
            if(m) if(m.firstChild.textContent === "Ten. Million. Questions. Let's celebrate ") {
                    clearInterval(interval);
                    document.head.innerHTML += '<style>#system-message { display: none; }</style>';
            } else localStorage['SOHideSystemMessage_done'] = true;
        });
    } else if(!localStorage['SOHideSystemMessage_ack']) {
        alert('The banner is gone, you should be able to remove this script now.');
        localStorage['SOHideSystemMessage_ack'] = true;
    }
})();