v1 of implementing xkcd substituions: http://xkcd.com/1288/

sub.js

function replace(txt) {
  txt = txt.replace(/witnesses/gi, "dudes I know");
  txt = txt.replace(/allegedly/gi, "kinda probably");
  txt = txt.replace(/new study/gi, "tumblr post");
  txt = txt.replace(/rebuild/gi, "avenge");
  txt = txt.replace(/space/gi, "spaaace");
  txt = txt.replace(/google glass/gi, "virtual boy");
  txt = txt.replace(/smartphone/gi, "pokédex");
  txt = txt.replace(/electric/gi, "atomic");
  txt = txt.replace(/senator/gi, "elf-lord");
  txt = txt.replace(/car/gi, "cat");
  txt = txt.replace(/election/gi, "eating contest");
  txt = txt.replace(/congressional leaders/gi, "river spirits");
  txt = txt.replace(/homeland security/gi, "homestar runner");
  txt = txt.replace(/could not be reached for comment/gi, "is guilty and everyone knows it");
  return txt;
}

function replace_all_on_page() {
  function recurse(element)
  {
      if (element.childNodes.length > 0) 
          for (var i = 0; i < element.childNodes.length; i++) 
              recurse(element.childNodes[i]);

      if (element.nodeType == Node.TEXT_NODE) {
          window.el = element
          element.textContent = replace(element.textContent);
      }
  }
  var html = document.getElementsByTagName('html')[0];
  recurse(html);
}
replace_all_on_page();

Iterating through document.all might be better (benchmarking required)

will this work as a bookmarklet?