Adds HTML5 placeholders when they are not supported by your browser. Uses jQuery.

placeholder.js

/**
 * HTML5 Placeholders for old browsers
 * by Rutger Laurman
 *
 * Adds HTML5 placeholders to non-supported browsers with jQuery
 * Fork from Marc Görtz
 */
var HTML5Placeholders;

HTML5Placeholders = {
  addPlaceholders: function(props) {
    var config, key, testform;
    config = {
      "class": "placeholder"
    };
    for (key in props) {
      if (config.hasOwnProperty(key)) {
        config[key] = props[key];
      }
    }
    testform = $("<form><input type=\"text\"></form>");
    $.extend($.support, {
      placeHolder: !!($("input", testform)[0].placeholder !== undefined || $("input", testform)[0].placeHolder !== undefined)
    });
    if (!$.support.placeHolder) {
      $("html").addClass("no-placeholder");
      return $("input[placeholder], textarea[placeholder]").each(function() {
        var placeholder;
        placeholder = $(this).attr("placeholder");
        $(this).bind({
          focus: function() {
            if ($(this).val() === placeholder) {
              $(this).val("");
              $(this).removeClass(config["class"]);
            }
            return $(this).attr("data-focused", "yes");
          },
          blur: function() {
            if ($(this).val() === "") {
              $(this).val(placeholder);
              $(this).addClass(config["class"]);
            }
            return $(this).removeAttr("data-focused");
          }
        });
        if ((($(this).val() === "") || ($(this).val() === placeholder)) && (!$(this).attr("data-focused"))) {
          $(this).val(placeholder);
          return $(this).addClass(config["class"]);
        }
      });
    } else {
      $("html").addClass("placeholder");
    }
  }
};

$(document).ready(function() {
  return HTML5Placeholders.addPlaceholders();
});

15-11-2012 Updated with HTML classes to use in CSS