about values in a form.. w Ajax of course

You might have to use the :frequency option to observe_field. If you
are modifying the field with javascript it might not cause the browser
to through the event's which is how the default observe function works
without a frequency option. I ran into this problem with submitting a
form through javascript and it wasn't catching my onSubmit handler
event to cancel it and do a ajax call. But try it with the frequency
option.