A few days ago I spent some time scratching my head about how to use the prototype observe_field helper to update fields in the browser DOM when a checkbox is clicked without a remote server request.
The function option of observe_field (also observe_form) is not well documented, and google didn't help much.
I worked it out and just blogged about what I found at http://talklikeaduck.denhaven2.com/articles/2007/10/07/observing-fields-with-rails-in-the-browser
And I submitted a rails trac ticket with a doc patch to improve the documentation. http://dev.rubyonrails.org/ticket/9808
Consider this a plea for folks to verify the patch.