by Tim Hunt.
mod_forumng used to do a completely ajax-y mform, when you replied to a post. However, sam chaged it to just display the new post form in an iframe. The iframe is still added using Ajax, without a full page reload. (Complexities to do with using the file-picker made that a much more robust option.)
mod/quiz/edit.php has some forms that appear in ajax-y pop-ups. They work be rendering the form into the page, but hidden, as part of the initial page load. Then JS just has to reveal the form.