update element after in_place_editor ?

Rails has a helper that generates the controller method for you. You would add:

   in_place_edit_for :item, :title

Also, in_place_editor_field generates an Ajax.Updater, so the render
:update is not really appropriate. From the docs for in_place_editor,

"..the action on the server should process the value and return the
updated value in the body of the reponse"

Using in_place_edit_for in your controller takes care of that for you.