<%=  f.check_box :foobar, :onchange => "
			if(this.checked==true) {
				$('binding_pp').visualEffect('Appear');
				Element.update('phone_number_select', '#{render :partial => \"foobar1\"}')
			} else {
				$('binding_pp').visualEffect('Fade');
				Element.update('phone_number_select', '#{render :partial => \"foobar2\"}')
			}
		" %>

# foobar1 and foobar2 both render something like

   f.select :phone_number, ....

# but with different options. I cannot use display:none CSS
# because then there would be two "phone_number" objects
# in the submitted form data, even if one is invisible, and
# this would cause wrong data to be submitted (potentially).