Soon, we still don't know when ... Authenticator holders will need to enter this code in the forums if they want to write. Personally, I found it strange that the Authenticator was not necessary in the forums so it is a good measure.
As part of our continued dedication to account security, players using an authenticator on their account will be prompted the first time to log into the forum and periodically thereafter. This way, even if the account is compromised, without the authenticator, they won't be able to use your login credentials. This additional security measure helps to better protect players to all services accessed by logging in with their account and password.
We would like to urge all players who are not yet using one to consider linking an authenticator to help protect themselves and their accounts. Information on how to obtain an authenticator as well as additional security information can be found at: http://www.battle.net/security/
Are you planning to acquire an authenticator?