There's an addon called Group Calendar that sets up a syncrhonized calendar for the guild. We're using it as a method of signups (if people would actually sign up on it...) but it could easily work as a vacation notifier. Some people just can't get it to work, though. I don't know if it's being difficult or if they're a little less than addon savy.
http://www.wowinterface.com/download...fo.php?id=5539