Because The New Yorker does it...

Diaeresis.js

(function() {
  var words = [
    "Boötes",
    "Brontë",
    "caïquejee",
    "Chloë",
    "continuüm",
    "coöperate",
    "coöperation",
    "coöperating",
    "coöperative",
    "coöpt",
    "coördinate",
    "coördinated",
    "coördinating",
    "coördination",
    "coördinator",
    "coördinators",
    "Creüsa",
    "daïs",
    "faïence",
    "langue d'oïl",
    "naïf",
    "naïve",
    "naïveté",
    "Noël",
    "noöne",
    "oöcyte",
    "oölogy",
    "preëminent",
    "preëminently",
    "preëmpt",
    "preëmption",
    "preëmptive",
    "reëlect",
    "reëlected",
    "reëlecting",
    "reënter",
    "reëntered",
    "reëntering",
    "reëstablish",
    "reëstablished",
    "reëstablishing",
    "zoölogy"
  ];

  var diacriticsMap = {
    "ä": "a",
    "ë": "e",
    "ï": "i",
    "ö": "o",
    "ü": "u"
  };


  function forEachDiacritic(fn) {
    for(var diacritic in diacriticsMap) {
      if (diacriticsMap.hasOwnProperty(diacritic)) {
        fn(diacritic, diacriticsMap[diacritic]);
      }
    }
  }

  function removeDiacritics(text) {
    forEachDiacritic(function(diacritic, equivalent) {
      text = text.replace(new RegExp(diacritic, 'g'), equivalent);
    });

    return text;
  }

  // gets fancy so that we can preserve word capitalization
  function addDiacritics(text) {
    for (var i = 0; i < words.length; i++) {
      var diacriticWord = words[i],
          plainWord = removeDiacritics(diacriticWord);

      text = text.replace(new RegExp(plainWord, 'gi'), function(match) {
        forEachDiacritic(function(diacritic, equivalent) {
          var regex = new RegExp(diacritic, 'g');
          while ((charMatch = regex.exec(diacriticWord)) !== null) {
            match = match.substr(0, charMatch.index) + diacritic + match.substr(charMatch.index + diacritic.length);
          }
        });

        return match;
      });
    }

    return text;
  }

  function forAllTextNodes(fn, element) {
    element = element || document.body;
    var nodes = element.childNodes;
    for (var n = 0; n < nodes.length; n++) {
      if (nodes[n].nodeType === Node.TEXT_NODE) {
        fn(nodes[n]);
      } else {
        forAllTextNodes(fn, nodes[n]);
      }
    }
  }

  forAllTextNodes(function(node) {
    node.textContent = addDiacritics(node.textContent)
  });

})();

http://gistlet.com/?gist=5021506