There have been rumors on this topic since the day is drawing near.

There's green poster saying that rested xp will be rest as you upgrade your account/the launch day, while it will again accumulate afterwards.

http://forums.worldofwarcraft.com/thread.html?topicId=62540657&sid=1
http://forums.worldofwarcraft.com/thread.html?topicId=25171669431&sid=1

and there're people saying that it won't reset and you need to learn how to save rested xp at 80.

http://forums.worldofwarcraft.com/thread.html?topicId=27495544059&sid=2000
http://forums.wow-europe.com/thread.html?topicId=14574718404&sid=1&pageNo=2

So what's the real deal here?

Rested experience will be reset at launch. You will start at 0 rested.