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Google provides a set of numerical codes for you to print out and store in case you lose your phone. They're all one-time use, and allow you to get in and change the settings.

Does that mean that if ANY of them are used, the rest are invalidated? Or just that any one of them may be used once?

You get 10 codes per generation(and can regenerate them whenever you want), and each code can be used once.

You can use an alternate number to have a text message sent to you and there are printable one-time pads. The one-time pads have come in handy for me, because I always let the battery on my phone die.

The recommended way is to use the Google Authenticator app - available on Android, iOS, and BlackBerry devices - doesn't require an Internet connection, mobile service, or a data plan to generate verification codes.

http://www.google.com/support/accounts/bin/answer.py?answer=...

I'm using it on my iPod.

In addition to the one time pad, you can specify other phone numbers. The system will call these phones and read the code aloud.