To help keep 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 setup 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 reattach it if it's already associated with another Battle.net account.
For more information on the Battle.net authenticator and mobile authenticator application (available for free with many mobile carriers), please visit http://us.blizzard.com/support/article/blizzardauth