For the time being, here's a quick and dirty bookmarklet (or easily saved for Greasemonkey):

javascript:(function(){
  $("#sidebar").hide();
  $("#post-editor").width(1360);
  $(".community-option").css("float", "left");
  $("#wmd-container").css("float", "left");
  $("#wmd-preview").css("clear", "none")
    .css("margin-left", "20px")
    .css("float", "left"); 
})()

(Only quickly tested in Safari and Firefox on a Mac. I guess things would look much better when widening the whole page.)