Sneaky Abstractions

Of course, you could also avoid the constructor altogether and use a closure instead, favoring encapsulation over memory consumption.

Mysite = {};

Mysite.posts = $$('.post').map(function(element) {
  return {
    markRead: function(){
      element.addClassName('read');
    },
    markUnread: function(){
      element.removeClassName('read');
    },
    isRead: function(){
      return element.hasClassName('read');
    }
  };
});