To help keep our players' Battle.net accounts as secure as possible, we've recently changed how Battle.net authenticators can be used. Going forward, Battle.net authenticators can now only be associated with one Battle.net account at a time. No changes are being made to how many game licenses a single Battle.net account can support. You can still have multiple World of Warcraft accounts under a single Battle.net account, for instance, and all game licenses linked with a Battle.net account will still be protected if an authenticator is in use.
Those of you who currently have more than one Battle.net account associated with a single authenticator will be able to maintain your existing set-up without needing to do anything. This change will only affect new authenticator attachments. But, if at some point you decide to detach the authenticator from any of your Battle.net accounts for any reason, you won't be able to re-attach it if it's already associated with another Battle.net account.
For more information on the Battle.net authenticator and mobile authenticator application (which is available for free with many mobile carriers), please visit http://eu.battle.net/support/en/article/912
Why didn't I get an Authenticator prompt this time?
If you use an authenticator – and we hope you do – you may soon notice that an authenticator prompt may not appear with every login. We've recently updated our authentication system to intelligently track your login locations, and if you're logging in consistently from the same place, you may not be asked for an authenticator code. This change is being made to make the authenticator process less intrusive when we're sure the person logging in to your account is you, while still offering the same level of security.
If you'd prefer this prompt every login, there is now a check-box to enable this feature. Log into Account Management . choose Security Options from the top links. The next page for an account with an active authenticator is Manage Security Options, and you'll see Active: with a picture of your authenticator. The checkbox to enable this feature is directly under Remove Authenticator and Battle.net Authenticator FAQ.
We hope to continue improving the authenticator system to ensure the same or greater security, while improving and adding features to make having one a more user friendly experience. If you don't already have a Battle.net Authenticator attached to your account, don't wait until it's too late http://eu.battle.net/en/security/checklist