Using this in Rails v4.0.0.rc1 and Bootstrap v2.3.1

_flash.html.erb

<% [:notice, :error, :alert, :success].each do |level| %>                 
  <% unless flash[level].blank? %>                              
    <div class="alert alert-<%= flash_class(level) %>">         
      <a class="close" href="#">×</a>                           
      <%= content_tag :p, flash[level] %>
    </div>                                                      
  <% end %>                                                     
<% end %>                                                       

application_helper.rb

module ApplicationHelper
  def flash_class(level)
    case level
    when :notice then "info" #blue
    when :error then "error" #red
    when :alert then "warning" #yellow
    when :success then "success" #green
    end
  end
end