I logged in today to do some herb gathering on my scribe, and logged back out. When I went to log in later to update all of my add-ons, I wasn't prompted to enter my authenticator code. I logged out and back in again, and still no authenticator prompt. I, of course, freaked, thinking that I was in the middle of getting my account stolen, the bad guys had somehow taken the authenticator off of my account, and they would be changing my password any second now.
I logged in to Battle.net to put the authenticator back on my account and change my password, and they prompted me to enter my authenticator code this time. I calmed down and searched the forums, and it turns out that Blizzard changed the way authenticators work about a week ago. If you're logging in from a computer that you've logged in from before, and you entered your authenticator then, you may not be prompted for your authenticator every time you log in again. They're trying to make authenticators less intrusive, and not pester you every single time.
More info: http://us.battle.net/wow/en/forum/topic/2674529793
Glad you posted this, I would have freaked when I finally come back.. woulda thought my account got stolen.