jimmywalesanyware.user.js

// ==UserScript==
// @name JimmyWalesAnyware
// @namespace www.dolpen.net
// @include http://*
// @include https://*
// @exclude http://wikimediafoundation.org/*
// @exclude http://ja.wikipedia.org/*
// @exclude http://en.wikipedia.org/*
// @require http://jqueryjs.googlecode.com/files/jquery-1.3.2.min.js
// ==/UserScript==
(function(){
 var url='http://wikimediafoundation.org/wiki/Special:LandingCheck?landing_page=WMFJA1&language=ja&country=JP&utm_source=2010_JA1_Banner3&utm_medium=sitenotice&utm_campaign=fridayOpening';
 var box=$('<div></div>').css({
  'position':'absolute',
  'width':'100%',
  'zIndex':'65535'
 });
 var contents=$('<div></div>').css({
  'background-image':'url("http://upload.wikimedia.org/centralnotice/images/Jimmy-window-light.jpg")',
  'height':'172px',
  'background-position':'right top',
  'background-repeat':'no-repeat',
  'width':'100%',
  'margin':'0',
  'padding':'0',
  'font-size':'xx-large'
 }).append();
 var text=$('<div></div>').css({
  'padding':'1em',
  'color':'#333333',
  'cursor':'pointer',
  'font-family':'helvetica,arial,monospace',
  'font-size':'28px',
  'font-weight':'normal',
  'line-height':'1.125em',
  'text-align':'left'
 }).html('ウィキペディア創設者<br>ジミー・ウェールズからの<br>メッセージをお読みください');
 var btn=$('<div></div>').click(function(){
  object.hide();
 }).css({
  'font-size':'0',
  'line-height':'0',
  'position':'absolute',
  'width':'100%', 
  'margin':'0',
  'padding':'0',
  'text-align':'right'
 }).append(
  $('<a></a>').attr('href','#').append(
   $('<img />').attr('src','http://bits.wikimedia.org/skins-1.5/common/images/closewindow.png').css({'margin':'3px 3px 0 0','padding':'0','border':'0'})
  )
 );

 var banner=$('<a></a>').css({'display':'block','position':'absolute','width':'100%','text-decoration':'none'}).attr('href',url).hover(
  function(){text.css('text-decoration','underline');},
  function(){text.css('text-decoration','none');}
 ).append(
  contents.append(text)
 );
 var object=box.append(banner).append(btn);
 $('body').prepend(object);
})();

What exactly does this script do?