The forums showed up with a special note as the real money AH comes closer:
Battle.net Balance and Authenticator Update
With the introduction of the real-money auction house, account security will become more important than ever. To help ensure that players have a positive experience when using the real-money auction house, we’ve made some adjustments to how players can use and access their Battle.net Balance.
Starting today, in order to add to your Battle.net Balance, players will be required to have a Battle.net Authenticator or Battle.net Mobile Authenticator attached to their Battle.net account. For clarity, this means you’ll need to have an Authenticator to add to your balance via Battle.net Account Management or to send the proceeds of your real-money auction house sales to your Battle.net Balance.
Please note that players who previously added Battle.net Balance to their account prior to this change will be able to use it to make eligible purchases on Battle.net and in the auction house without attaching an Authenticator. However, an Authenticator will be required to add to your balance in the future, as explained above.
I applaud their decision as this ensures that your money stays your money.
There are free authenticators available on both Android and iOS along with the physical authenticators. There’s no reason for you not to have one at this point for your battlenet account.