Battle.net authenticators limited to one account
Blizzard is changing up the security on their authenticators a bit. This isn't a major change and shouldn't affect that many people. Starting now, if you happen to have multiple Battle.net accounts (not multiple WoW accounts under one Battle.net account), then each account must have its own authenticator. This means if you have separate Battle.net logins for zergrush@somedomain.com and taurenfever@example.com and you want to use an authenticator, you'll need to buy two. If you've just got taurenfever@example.com and all of your games are under that Battle.net login, then you're perfectly fine.
This is not retroactive. If you already have two accounts linked to a single authenticator, everything will still work as it does right now until you unlink that authenticator. The full blue post detailing the changes is behind the cut below.
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
Like I said at the beginning, this isn't something what will impact too many people, as a majority of players will only have a single Battle.net account, and all existing authenticator setups are grandfathered in.