I’ve been using this a lot in one of our current projects.. it comes in pretty handy. just create a

<div id="message" style="display: none"></div>

# this goes in your application_helper.rb
# call it from your layout like this:
#     <%= flash_for :error %>    
#     <%= flash_for :notice %>
#     <%= flash_for :success %>

  def flash_for symbol
    # <div id="flash_error" class="message-block error">
    #   text
    # </div>
      content_tag( :div, flash[symbol].to_s, :id => "flash_#{symbol}", :class => "message-block #{symbol}",
                   :style => flash[symbol] ? nil : 'display: none' )
  end

# this goes in your controller

   def login #http get, ajax post
    redirect_to home_url( current_user ) and return if user_logged_in? #already logged in?
    if request.xhr?
      raise "Please enter a valid email address." unless params[:user][:email] =~  /^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i
      raise "Please enter a password!" if params[:user][:password].blank?

      if user = User.authenticate( params[:user][:email], params[:user][:password] )
        ::ActionController::Base.session_options[:session_expires] = params[:user][:remember] ? 2.weeks.from_now : nil  # remember me for 2 weeks if checked
        login_user user
        ajax_redirect_to home_url(current_user)
      else
        session[:forgotten_email] = params[:user][:email]          
        raise "Incorrect email address or password." 
      end
    end
  rescue Exception => e
    handle_ajax_error e
  end

# this goes in application.rb
protected
  def ajax_redirect_to url
    url = url_for( url ) if url.is_a? Hash
    render :update do |page|
      page << "window.location = '#{url}'" 
    end
  end

  def handle_ajax_error e
    render :update do |page|
      page.flash_then_fade :error, "Error: <nobr>#{e.message}</nobr>" 
      page << "Element.scrollTo('flash_error');" 
    end
    logger.error "\n\n#{e.message}" 
    logger.info e.backtrace.join("\n")
  end  

# this goes in /lib directory in a file that gets loaded in environment.rb
module ActionView
  module Helpers
    module PrototypeHelper
      class JavaScriptGenerator
        module GeneratorMethods

          def flash_then_fade symbol, message, seconds=0, fadeout=1
            # update message text
            replace_html "flash_#{symbol}", message
            # hide all notices except current
            (%w[ error success notice ] - [ symbol.to_s ]).each do |sym|
              hide "flash_#{sym}" 
            end
            dom_id = "flash_#{symbol}" 
            visual_effect :highlight, dom_id, :duration => 2.0
            show dom_id
            if seconds.to_f > 0
              delay( seconds ) do
                visual_effect :fade, dom_id, :duration => fadeout
              end
            end
          end

        end
      end
    end
  end
end