def countdown_field(field_id,update_id,max,options = {})
    function = "$('#{update_id}').innerHTML = (#{max} - $F('#{field_id}').length);"
    count_field_tag(field_id,function,options)
  end
 
  def count_field(field_id,update_id,options = {})
    function = "$('#{update_id}').innerHTML = $F('#{field_id}').length;"
    count_field_tag(field_id,function,options)
  end
 
  def count_field_tag(field_id,function,options = {}) 
    out = javascript_tag function
    out += observe_field(field_id, options.merge(:function => function))
    return out
  end

<input id="message">
<p>You have used <span id="counter">...</span> letters.</p>

<%= count_field('message','counter') %>