The new iPhone 3G S will be available this Friday, and to prepare for that a new operating system for both iPhone and iPod Touch are available; version 3.0. If you're planning on upgrading your OS, you'll want to back up your Mobile Authenticator to avoid potential issues with getting it installed again.
Slorkuz posted some tips on it, so check them out below. Note that if you want to update your iPod Touch, it will cost $10, but it's free for iPhone users.
Quote from: Slorkuz (Source)
If you use the Battle.net Mobile Authenticator and intend to update your iPhone or iPod touch to iPhone OS 3.0, we would like to highly recommend that you first back up your iPhone or iPod touch to your computer using iTunes prior to performing the update. By taking this important precaution, you will be able to quickly restore your previous install of the Battle.net Mobile Authenticator if you encounter any issues during the update process.
For more information, refer to the Battle.net Mobile Authenticator FAQ here: http://eu.battle.net/bma