Originally posted on 9to5Mac:
Via the Wall Street Journal, an Apple spokesperson fleshes out some of the finer details surrounding the fingerprint sensor and Touch ID. To use Touch ID, it is mandatory to also set up a passcode. This acts as a fallback in case the fingerprint sensor fails temporarily or experiences a permanent hardware fault. iOS may necessitate a passcode under some other conditions, as well.
Only that passcode (not a finger) can unlock the phone if the phone is rebooted or hasn’t been unlocked for 48 hours. This feature is meant to block hackers from stalling for time as they try to find a way to circumvent the fingerprint scanner.