In an effort to step up security of Battle.net accounts around the world,
Blizzard is implementing a new change to how their Battle.net authenticators work -- most notaby being the fact that Battle.net authenticators can now only be associated with one Battle.net account at a time.
Luckily, for those of you who have one authenticator bound to a multiple Battle.net accounts, you will not be affected, but should you detach this authenticator from any of your Battle.net accounts, Blizzard warns you that you will be unable to re-attach it if it is already bound to a different account.
All in all, this just means that from now on, one authenticator to one Battle.net account.
More...