Many people, myself included, are excited about the iPhone OS 3.0 update, which is released tomorrow for all iPhone and iPod Touch owners, though Touch users have to pay for it. If you use the Blizzard Mobile Authenticator app, though (and there's little reason not to; it is free, and extra security never hurts), you should take one precaution first.
Every so often, even the best-planned upgrades go awry. In case your iPhone 3.0 upgrade breaks and you have to restore the iPhone, you should make sure to write down the serial number of your Mobile Authenticator ahead of time (or take a screenshot of it and transfer it to your computer).
The serial number is found on the Setup screen of the Authenticator, and Blizzard will need this number from you if you need to detach the Authenticator from your account (because your iPhone needed to be restored the Authenticator got a new serial number).
For the majority of users, the upgrade process will probably go seamlessly and you won't need this. However, if you do, you'll be happy you have it.