## View:

<%= text_field 'user', 'username', { :autocomplete => "off" } %>
<%= observe_field "user_username",
      :url => account_url(:action => "validate"),
      :with => "user[username]",
      :frequency => 0.25 %>
<%= content_tag "div", "", :id => "error_username", :style => "display:none;" %>

## Controller:

  def validate
    if request.xhr?
      @user = User.new( params[:user] )
      # Try to validate the user (this will add errors)
      @user.valid?
      render :action => "validate.rjs"
    else
      render :nothing => true
    end
  end

## validate.rjs

attrs = params[:user].map {|k,v| k }

attrs.each do |attr|
  if @user.errors.on(attr)
    errors = @user.errors.on(attr)
    page["error_#{attr}"].replace_html @user.class.human_attribute_name(attr) + " " + ( errors.is_a?(Array) ? errors.first : errors )

    page << "if ($('error_#{attr}').style.display == 'none') {"
      page.visual_effect :blind_down, "error_#{attr}", :duration => 0.25
    page << "}"

  else
    page << "if ($('error_#{attr}').style.display != 'none') {"
      page.visual_effect :blind_up, "error_#{attr}", :duration => 0.25
    page << "}"
  end
end