Now, a single authenticator or installation of a mobile authenticator can only be attached to a single Battle.net account. If you already had multiple Battle.net accounts tied to a single auth, you will not need to purchase additional devices and it will continue to function as it has.
This change applies only to new authenticators, but those that remove an authenticator from a Battle.net account will not be able to re-associate it with another account if it still has at least one account linked.
This should help reduce some of the account theft, or at least make it more cumbersome for thieves as they will have to get a new auth for each account they attempt to steal.
If you have not already added an authenticator to your Battle.net account, I recommend you do so, especially if you have a World of Warcraft account. Free versions are available as applications for iOS and Android.