Well, since there have been asked alot on the forums about the "Blackrock Caverns Entrance discovered"-thing, I decided ask a GM if you had to rediscover it or not. And he replied that you should be able to use it without discovering it again, so this is definitely a advantage if you are going for quick 85, or realm first.
GM Ticket:
http://img508.imageshack.us/f/gmrespond.png/
Sorry if someone already confirmed this, haven't found anything about it.
EDIT: Entrance is located here: http://www.youtube.com/watch?v=dTgdmT0WBp4 for those who don't know .