I wonder if Apple has considered using voice as an auth method? (Answer: probably). If I'm not mistaken our voices are as unique as fingerprints and could be used to identify an individual. Not being an expert in this area though, perhaps there are cases where an individual's voice may not be unique enough to always act as a sufficiently unique identifier, plus voice playback would be too easy for those trying to circumvent the lock.
If Apple could implement something like that, it could use the iPhone's mic and omit the need for fingerprint or visual cues. This would, of course not be available to individuals who have lost the use of their voice or have compromised vocal cords, and also be inconvenient for cases where using your voice would be disruptive.
So I guess I've answered my own question - probably ain't happening, although it would be a cool option for those interested in this as a way to unlock.
The mics on smartphones are lame. They are one reason Siri has difficulty understanding you.