Scott Fleckenstein has developed a plugin for Ruby on Rails called Exceptional that 'brings exception handling into the world of Rails filters'. The code speaks for itself on this one:

class SampleController <  ApplicationController
  handles AccessDeniedError,
    :with => :access_denied,
    :only => [:supah_sekret_akshun]

  def supah_sekret_akshun
    ...
    raise AccessDeniedError
    ...
  end

  private
  def access_denied
    render :text => 'no, u kant'
  end
end

There's a bit more to it than that, so check out the official plugin site for examples and how to download the plugin.