I modified rstakeout.rb a bit to allow platform-dependent notification via Growl or Snarl. See Pastie 152014
Update: Pastie nr. 177066 works in Linux too.
Update 2: It’s likely that http://www.nomedojogo.com/2008/05/14/the-autotest-notification-gem/ contains a much better tested implementation, so maybe you should use that instead.