downcase.rb

class Downcase
  def initialize(app)
    @app = app
  end
 
  def call(env)
    # execute the request using our Rails app
    status, headers, body = @app.call(env)
  
    if status == 404 and url = downcased_url(env['REQUEST_URI'])
      # "Moved permanently" to new url
      [301, {"Location" => url}, 'Redirecting']
    else
      # return the result from rails
      [status, headers, body]
    end
  end
   
  def downcased_url(path)
    new_path = path.downcase
    if new_path == path
      # path's are the same
      return false
    else
      return new_path
    end
  end
end

environment.rb

Rails::Initializer.run do |config|
  # <snip>
  config.middleware.use 'Downcase'  
end