From the editable homepage (http://www.appelsiini.net/projects/jeditable):
Note that function must return string. Usually the edited content. This will be displayed on page after editing is done.
edit_menu_item doesn't return a string - it would seem that you can get around that by returning value, then if you need to update the actual contents, do so in the success callback of your ajax request. You might want to also add an error callback, just in case.
An alternative, which is probably a bad idea, would be to mark your ajax call as synchronous (async : false) and looking at the responseText, but that would lock up the page until your request completes.
Dan McCormick
2010-03-24 00:47:42