localStoragePolyfill.js

// Adds Support for localStorage to IE5-IE7 using userData 
// <http://msdn.microsoft.com/en-us/library/ms531424(VS.85).aspx>
// See also <http://caniuse.com/#feat=namevalue-storage>
if(typeof window.localStorage === 'undefined') (function () {
    var userdataKey = 'localStorage',
        userdata = document.createElement('b');

    userdata.style.display = 'none';
    userdata.style.behavior = 'url("#default#userData")';

    document.body.appendChild(userdata);

    userdata.load(userdataKey);
    
    window.localStorage = {
      setItem: function (key, val) {
        userdata.setAttribute(key, val);
        userdata.save(userdataKey);
      },

      removeItem: function (key) {
        userdata.removeAttribute(key);
        userdata.save(userdataKey);
      },

      getItem: function (key) {
        return userdata.getAttribute(key);
      }
    };
  }
})();

This probably needs better error handling. This also doesn't make the localStorage api support object storage (JSON) which is a feature I love. This is just a simple polyfill so you have to make sure you're only storing strings. Also you have to use .getItem() and .setItem() or this polyfill wont work.